. The shell around the nucleus can be numbered from near to the nucleus as 1, 2, 3, 4, 5 or represented by the letters, K, L, M, N, O,​

He numbered these orbits due to their location.

Explanation:

Scientist Neil Bohr in his atomic model describes that electrons are moving with high velocity in circular paths around the nucleus is called shells or orbits. As long as electrons are in their particular orbit their energy is constant. He named those orbits as K- shell, L- shell and M- shell etc. When the electron absorb energy, it is able to jump to the next shell and comes back to its original shell when it loses energy. He also give numbers i.e. 1,2,3,4 to the shells due to their nearness to the nucleus. Shell 1 is near to the nucleus whereas shell 4 is farther from the nucleus.

4d^55s^1 ( third choice)