サンプルコード

index.php

<!DOCTYPE html>
<html lang="ja">
<head>
	<title>test</title>
	<link rel="stylesheet" type="text/css" href="test.css">
</head>
<body>
<ul>
	<li class="choice">A1</li>
	<li class="choice">A2</li>
	<li class="choice">A3</li>
	<li class="choice">A4</li>
</ul>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<script src="test.js"></script>
</body>
</html>

test.css

body {
  font-size: 28px;
  font-family: font-family: "Avenir Next", Verdana, "ヒラギノ角ゴ Pro W3", "Hiragino Kaku Gothic Pro", "游ゴシック", "Yu Gothic", "メイリオ", Meiryo, Osaka, sans-serif;
}
.selected {
	color: red;
}

test.js

$(function() {
	$('.choice').on('click', function() {
		// $(this)は.choiceを指す変数
		var $selected = $(this);
		$selected.addClass('selected');
	})
})

結果