Which HTML should you use?

You are developing a customer web form that includes the following HTML.
<input id=”txtValue”/>
You need to change the HTML markup so that customers can enter only a valid three-letter country
code.
Which HTML should you use?

A.
<input id=”txtValue” type=”code” />

B.
<input id=”txtValue” type=”text” pattern=” [A-Za-z] {3}” />

C.
<input id=”txtValue” type=”text” required=”required”/>

D.
<input id=”txtValue” type=”country” />

Explanation:

Reference:
http://www.w3schools.com/html5/att_input_pattern.asp