JSFiddle - React, Tailwind, and code Playground

HTML

<select class="dropdown" id="category" name="cat">
  <option value=""></option>
  <option value="1">Entertainment</option>
  <option value="2">Sound</option>
  <option value="3">Peripherals</option>
  <option value="4">Storage</option>
  <option value="5">Power</option>
  <option value="6">Network</option> 
</select> 
 

<select class="dropdown" name="subcat" id="subcat">
  <option value=""></option>
  <option class="cat0 "  value="1">Entertainment</option>
<option class="cat0 "  value="2">Sound</option>
<option class="cat0 "  value="3">Peripherals</option>
<option class="cat0 "  value="4">Storage</option>
<option class="cat0 "  value="5">Power</option>
<option class="cat0 "  value="6">Network</option>
<option class="cat1 "  value="7">Tablets</option>
<option class="cat1 "  value="8">Media Players</option>
<option class="cat1 "  value="9">DVD Players</option>
<option class="cat1 "  value="10">Photo Frames</option>
<option class="cat2 "  value="11">PSYC Audio</option>
<option class="cat2 "  value="12">Headsets</option>
<option class="cat2 "  value="13">Mp3 Players</option>
<option class="cat2 "  value="14">Speakers</option>
<option class="cat3 "  value="15">AIO Card Readers</option>
<option class="cat3 "  value="16">Cables</option>
<option class="cat3 "  value="17">Keyboards</option>
<option class="cat3 "  value="18">Mouse</option>
<option class="cat3 "  value="19">PCI Cards</option>
<option class="cat3 "  value="20">USB Hubs</option>
<option class="cat3 "  value="21">VoIP</option>
<option class="cat4 "  value="22">DVD Accessories</option>
<option class="cat4 "  value="23">Enclosures</option>
<option class="cat4 "  value="24">iPad Accessories</option>
<option class="cat4 "  value="25">Optical Media</option>
<option class="cat4 "  value="26">PC Cases</option>
<option class="cat4 "  value="27">Photo Papers</option>
<option class="cat5 "  value="28">Chargers</option>
<option class="cat5 "  value="29">Inverters</option>
<option class="cat5 " ...

JavaScript

jQuery.fn.toggleOption = function( show ) {
    jQuery( this ).toggle( show );
    if( show ) {
        if( jQuery( this ).parent( 'span.toggleOption' ).length )
            jQuery( this ).unwrap( );
    } else {
        jQuery( this ).wrap( '<span class="toggleOption" style="display: none;" />' );
    }
};

$('#category').on('change', function(){
    var val = $(this).val();
    /* $('#subcat option').prop('disabled', function(){
        return !$(this).hasClass('cat' + val)
    }); */
    $('#subcat option').each(function(){
        var $option = $(this);
        $option.toggleOption( $option.hasClass('cat' + val) );
    });
});