While "do" can be used in some contexts, it's not the standard way to reply to a request. Here's why:
* "I will" expresses a commitment to fulfill the request. It's a clear and direct affirmation.
* "Do" on its own is a bit vague and could be misinterpreted. It could mean you understand the request but aren't necessarily committing to doing it.
Example:
* Request: Could you please close the window?
* Response: I will.
This response clearly conveys that you will close the window.