[class.selected]="string=== AnotherString"


EXAMPLE <li [class.selected]="hero === selectedHero"> foo </li>

the code above is saying:

apply the class selected if the hero string is equals to the selectedHero string

in angular 1, you would use it like this:

ng-class="{ 'QuizImageChecked' : answerCheck[0]==option }"

or
data-ng-class="{ 'QuizImageChecked' : answerCheck[0]==option }"

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