<p class="choice">
        <label>
            <input type="radio" class="a" name="1"><span class="aFont">aaaaa</span>
        </label>
        <label>
            <input type="radio" class="b" name="1"><span class="bFont">bbbbb</span>
        </label>
        <label>
            <input type="radio" class="c" name="1"><span class="cFont">ccccc</span>
        </label>
        <label>
            <input type="radio" class="d" name="1"><span class="dFont">ddddd</span>
        </label>
    </p>
<script>
        $(document).ready(function(){
            $(".choice input[type = 'radio']").on("click",function(){
                if ($(".choice input:checked")){
                    $(this).parent().css("background" , "green").siblings().css("background","red");
                }
            });
        });
</script>

你選的 是input 不是label 注意 $(this).parent().css("background","green").siblings()

程式碼沒測試, 思路就是先移除所有背景色,再給點擊的那一個添加背景色, 你這個if判斷是不需要的

<script>
        $(document).ready(function(){
            $(".choice input[type = 'radio']").on("click",function(){
                   $(".choice input[type = 'radio']").css("background","");
                   $(this).css("background","red");
            });
        });
</script>