I am having a lot of trouble being able to solve the opacity challenge. In the code I currently have, the opacity is applied from the beginning (making spoilers of the result lol).

I can't get the ternary conditional to happen after the user chooses the right option. Any help? Thank!!!

//
//  ContentView.swift
//  GuessTheFlag
//
//  Created by Lucas on 10/11/21.
//

import SwiftUI

extension Image: ViewModifier {
    func flagImage() -> some View {
        self
            .clipShape(Capsule())
            .shadow(radius: 10)
    }
}

struct ContentView: View {
    @State private var showingScore = false
    @State private var scoreTitle = ""
    @State private var score = 0

    @State private var endGame = false
    @State private var questionAsked = 0

    @State private var finishTitle = ""

    @State private var countries = ["Estonia", "France", "Germany", "Ireland", "Italy", "Nigeria", "Poland", "Russia", "Spain", "UK", "US"].shuffled()
    @State private var correctAnswer = Int.random(in: 0...2)

    //Animation

    @State private var animationAmount = 0.0
    @State private var opacityAmount = 0.5

    var body: some View {
        ZStack {
            RadialGradient(stops: [
                .init(color: Color(red: 0.1, green: 0.2, blue: 0.45), location: 0.3),
                .init(color: Color(red: 0.76, green: 0.15, blue: 0.26), location: 0.3)
            ], center: .top, startRadius: 200, endRadius: 800)
                .ignoresSafeArea()

            VStack {
                Spacer()

                Text("Guess the Flag")
                    .font(.largeTitle.bold())
                    .foregroundColor(.white)

                VStack (spacing: 15){
                    VStack {
                        Text("Tap the flag of")
                            .font(.subheadline.weight(.heavy))
                            .foregroundStyle(.secondary)
                        Text(countries[correctAnswer])
                            .font(.largeTitle.weight(.semibold))
                    }

                    ForEach(0..<3) { number in
                        Button {
                            flagTapped(number)
                            withAnimation {
                                animationAmount += 360
                                opacityAmount = 1.0
                            }
                        } label: {
                            Image(countries[number])
                                .flagImage()
                        }
                        .rotation3DEffect(.degrees((number == correctAnswer) ? animationAmount : 0.0), axis: (x: 0, y: 1, z: 0))
                        .opacity((number == correctAnswer) ? opacityAmount : 0.5)
                    }
                }
                .frame(maxWidth: .infinity)
                .padding(.vertical, 20)
                .background(.ultraThinMaterial)
                .clipShape(RoundedRectangle(cornerRadius: 40))

                Spacer()
                Spacer()

//                Text("Question: \(questionAsked)") // Just to see the number
                Text("Score: \(score) ")
                    .foregroundColor(.white)
                    .font(.title.bold())
                Spacer()
            }
            .padding()
        }
        .alert(scoreTitle, isPresented: $showingScore) {
            Button("Continue", action: askQuestion)
        } message: {
            Text("Your score is \(score)")
        }
        .alert(finishTitle, isPresented: $endGame) {
            Button("Start again!", action: resetGame) // Change action
        } message: {
            Text("Great! Your final score is \(score)")
        }
    }

    func flagTapped(_ number: Int) {
            if number == correctAnswer {
                scoreTitle = "Correct!"
                score += 1
            } else {
                scoreTitle = "Wrong! That's the flag of \(countries[number])"
            }
            showingScore = true
    }

     func askQuestion() {
         if questionAsked == 9 {
             finishTitle = "Game Finish"
             endGame = true
         } else {
             countries.shuffle()
             correctAnswer = Int.random(in: 0...2)
             questionAsked += 1
         }
    }

    func resetGame() {
        score = 0
        questionAsked = 0
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

It has worked and I have also been able to improve my code thanks to your suggestions. Thank you very much for your very complete answer!