It’s simple to submit a form easily with the help of submit button, which will submit the data to process further. But in drop down item after selecting an option, we have to click submit button to submit form.
we can also  submit form automatically after selecting an option in drop down item using on change option action via JavaScript.

Submitting Form using Submit button

<form action="filename.php" method="POST">
<select name="sel">
<option> Option 1 </option>
<option> Option 2 </option>
<option> Option 3 </option>
<option> Option 4 </option>
</select>
<input type="submit" value="submit">
</form>

Submitting Form using onChange JavaScript

<form action="filename.php" method="POST">
<select name="sel" onchange="this.form.submit();">
<option> Option 1 </option>
<option> Option 2 </option>
<option> Option 3 </option>
<option> Option 4 </option>
</select>
</form>