JSFiddle - React, Tailwind, and code Playground

HTML

<select id="ma" class="single-option-selector" name="frequency">
    <option value="">Monthly</option>
    <option value="annual">Annual - 1 Month Free</option>
</select>
<br />
<br />
<br />
<br />
<a href="#" class="monthly-btn btn">
    <span id="AddToCartText">Select Monthly Plan</span>
</a>

<a href="#" class="annual-btn btn">
    <span id="AddToCartText">Select Annual Plan</span>
</a>

CSS

.annual-btn { display: none; }

JavaScript

$(function(){
    $('#ma').change(function(){
       var opt = $(this).val();
        if(opt == 'annual'){
            $('.monthly-btn').hide();
            $('.annual-btn').show();
        }else{
            $('.monthly-btn').show();
            $('.annual-btn').hide();
        }
    });
});