小编And*_*ack的帖子

$('#btn_25').on('click', function(e) {
   $('#my_selector option').attr('selected', false);
    $('#my_selector option[value=' + 25 + ']').attr('selected', true);
});

$('#btn_50').on('click', function(e) {
   $('#my_selector option').attr('selected', false);
    $('#my_selector option[value=' + 50 + ']').attr('selected', true);
});

$('#btn_100').on('click', function(e) {
   $('#my_selector option').attr('selected', false);
    $('#my_selector option[value=' + 100 + ']').attr('selected', true);
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select id="my_selector">
    <option value="25">Twenty Five</option>
    <option value="50">Fifty</option>
    <option value="100">One Hundred</option>
</select>

<button id="btn_25">25</button>
<button id="btn_50">50</button>
<button id="btn_100">100</button>