To select an HTML option by index:
JavaScript
$("#myColors").prop('selectedIndex', index);
To select an HTML option by value:
JavaScript
$("#myColors").val(Element value);
Html
<html>
<!-- heading -->
<head>
<meta charset="utf-8">
<title>Example</title>
</head>
<!-- page -->
<body>
<!-- connect the library jQuery -->
<script src="https://dir.by/example_lib/jquery/jquery-3.3.1.min.js"></script>
<!-- HTML Elements -->
<select id='myColors'>
<option value='10'>Red</optoin>
<option value='20'>Green</option>
<option value='30'>Blue</option>
</select>
<select id='myBooks'>
<option value='90'>Three Cats</optoin>
<option value='100'>Magician</option>
<option value='120'>Magician</option>
<option value='120'>Fairy tales</option>
</select>
<!-- Java Script -->
<script>
// the page has opened
$( function()
{
var elemSelect = $("#myBooks");
// select item by index
$("#myBooks").prop('selectedIndex', 2);
// 'Magician'
// select an item by value
$("#myColors").val(30);
// 'Blue'
});
</script>
</body>
</html>