According to the Pauli exclusion principle,
a) Two electrons can occupy the same orbital only because the electron spin can take on two
orientations.
b) Two electrons can never be measured within a distance Δx ≈ h/Δp of one another
c) Three electrons can occupy the same orbital only if the orbital size is greater than h
d) The electron of a hydrogen atom cannot come very close (< 0.01 Å) to the nucleus.
e) b. and c.

please explain because I have no idea on this question!

I would think your textbook would address the Exclusion Principle in enough detail so you can answer your own question. Which answer do you think is correct? Why? The choices are pretty basic.
Think it over, post back with your answer and we can give more help if you need it.

I think the answer is (a) because there could be two electrons with opposite spin in an orbital, and choices b-d are never stated in the book

Good job. I think you are absolutely correct. Answer b concerning the separation of two electrons would seem to be difficult to fit with the Heisenberg Uncertainty Principal. Answer c would be wrong beause only two electrons can occupy an orbital and then only if answer a permits it. As I said in an earlier answer, I think s electrons can actually be INSIDE the nucleus so I think answer d must be wrong. You did a good job.