. . . . . . . . I come in, sir? A. How B. May be C. May D. Should

The correct answer is “May I come in, sir?”.

“May I” is a polite way to ask for permission to do something. In this case, the speaker is asking permission to enter the room. “Sir” is a respectful way to address a man.

The other options are not as polite. “How” is a question word, and “may be” and “should” are modal verbs that can be used to express possibility or obligation. In this context, they would not be appropriate ways to ask for permission.

Here are some other examples of how to use “may I”:

• May I borrow your pen?
• May I have a glass of water?
• May I use your phone?

In each of these cases, the speaker is asking for permission to do something. The use of “may I” is a polite way to make the request.