Onclick, Highlight only one list item in an Unordered list. Javascript

Iam following a code to make a quiz.Iam building a function to highlight one list item and add a class to it.

enter code here
function chooseAnswer(){
    answer.querySelectorAll('li').forEach((option)=> { 
    option.addEventListener('click',()=>{
        if(answer.querySelector('.selected')){
            let activeAnswer=answer.querySelector('.selected');
            activeAnswer.classList.remove('selected') 
        }else
        option.classList.add('selected')
})

The code is working but i do not understand the if condition from the source. Can someone help?