Which orbital is the first to fill with electrons in a given principal energy level?
a. both d and f
b. s
c. f
d. p
e. both s and p

The right option is (B) S orbital

Each orbital can hold two electrons. One spin-up and one spin-down. The first principal energy level contains only an s sublevel; therefore, it can hold a maximum of two electrons.

The order in which the orbitals are filled is given below

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, 8s