Consider the last electron in B: Values for n,l,and ml of 2,0,0 cannot be used. Why not?

so is this the same answer as lithium. The ground stat cant have l=2 since it has no p electrons?

Are values of 3,0, 0 allowed? Why or why not?

I would say no for the same reason.

Are these correct?

2 answers

Close but not quite. 2,0,0 can't be used for the last electron in B because 2,0,0 has already been used for the last two electrons in Be (one electron with +1/2 ms and one with -1/2 ms). From your nomenclature the naming is n, l, m so n = 2, l = 0 (not 2 as you suggest) and it DOES have p electrons.
Your reasoning for 3,0,0 is correct.
so for 3,0,0 it would be because these numbers have already been used?