Answer:

The correct answer is – (4) 1s2 2s2 2p5 3s2

Explanation:

An excited state is a state when the valence electron has moved to some other higher energy orbital, from its ground state orbital. The ground state has a lower energy level or sublevel. In this case, the higher energy level orbit fills before the lower energy level.

In option 4, the last electron is filled in higher energy orbit 3s2 before filling the lower or ground energy level 2p5, in the ground state it would be 1s2 2s2 2p6 3s1 instead of 1s2 2s2 2p5 3s2.

Thus, the correct answer is option 4.