Hello @hashimtahir,
Let’s say we have this dropdown field:
[select your-options "Option 1" "Option 2" "Option 3"]
You can bold a specific option from this dropdown using the field name and option value as a CSS selector:
.your-options option[value="Option 2"] {
font-weight: bold;
}
Now the option you pointed should look like this:

If you want to disable a specific option, you must use some JavaScript code to achieve it. Let’s suppose that we want to deactivate option 3, you can do something this way:
(function( $ ) {
'use strict';
$(document).ready( function(){
var $option = $('option:contains("Option 3")');
$option.attr('disabled',true);
});
})(jQuery)
Now that specific option should not be clickable and look like this:

Best regards,
Yordan.