All of them except katrina helps her brother who is six years old with his homework is essential
Noun or verb
is that what your asking?
A. building tables,
because I'm not sure how that will exactly help you in studying. However, in my opinion, the other three seem able to work.
Sorry if this answer is kind of late, but this is the first time in ages since I've logged on. Have a good night and life though! :3