JSFiddle - React, Tailwind, and code Playground

HTML

<select size="1" id="rambo" name="comm-name" selected="selected" onchange="showhide(this)" >
        <option value="1" selected="selected">-Select Office-</option>
        <option value="2">HEAD OFFICE</option><option value="4">REGIONAL OFFICE</option>
        <option value="3">DIVISIONAL OFFICE</option><option value="6">BRANCH OFFICE</option>
        <option value="4">SERVICE CENTER</option><option value="8">EXTENSION COUNTER</option>
        <option value="5">Show div 9</option><option value="10">Show div 10</option>
    </select>
<div id="di21"> <select name="" id="rambo2">
        <option value="1">Select an option</option>
        <option value="2">Show div 4</option>
        <option value="3">Show div 5</option>
        <option value="4">Show div 6</option>
    </select> </div>

JavaScript

window.onload = function() { BindEvent(); }

    function BindEvent()
    {
       
        var elemToBind = document.getElementById ( "rambo" );
        elemToBind.onchange = function () { SetSel ( this ); }
    }
    function SetSel(elem)
    {
       
        var secondCombo = document.getElementById ( "rambo2" );
        secondCombo.value = elem.value;   
    }