Answer
Please see the work below.
Work Step by Step
We know that
a. Not allowed because the two electrons in 1s orbital should have the opposite spins.
b. Allowed because the electronic configuration is $1s^2 2s^2 2p^4$
c. Not allowed because 2s orbital can hold at the most two electrons having opposite spins.
d. Not allowed because the unpaired electrons in 2p orbitals should have the parallel spins.
