We are very busy . . . . . . . . weekdays. A. in B. on C. at D. of

The correct answer is: on.

The word “on” is used to indicate a specific day or date. In this case, the speaker is saying that they are very busy on weekdays.

The other options are incorrect because they do not indicate a specific day or date.

• The word “in” is used to indicate a period of time. For example, “We are very busy in the morning.”
• The word “at” is used to indicate a specific time. For example, “We are very busy at 9:00 AM.”
• The word “of” is used to indicate possession or association. For example, “We are very busy of weekdays.”

I hope this helps!