Whenever possible, write on a subject . a. that interests you. b. that you already know something about. c. that you have researched.
d. all of these.

d. All of these

Explanation:

When writing on a particular subject or topic, it's important to write on some things that interests you about what you're writing on, what you already know something about and what you have researched.

All the three options are vital if one wants to pay vivid attention to the subject that one is writing on. Having a previous knowledge gives one an advantage as one will be able to write on the subject well.

and whats your questoin