Which electronic level would allow the hydrogen atom to absorb a photon but not to emit it?
(a)- 1s
(b)- 2s
(c)- 3s
(d)- 2p

Hint:The arrangement or the pattern in which electrons are arranged within an atom is called the electronic configuration of that atom. The electrons are arranged in the basis of the energy of the levels, i.e. lower electronic levels are filled first followed by higher levels.

Complete step by step answer:
Before we proceed further, let us have an idea about what an orbital is and how the Aufbau principle comes in play here.

An orbital is a region present around the nucleus within which the probability of finding an electron is maximum. Orbitals are of different kinds namely s, p, d, f, varying in their shapes. The shape of the s-orbital is spherical, having the nucleus at its center. p-orbital is dumbbell-shaped and four of the five d orbitals are cloverleaf shaped. Lastly, the fifth d orbital is shaped like an elongated dumbbell with a doughnut around its middle.

Aufbau principle states that in the ground state of the atoms, the orbitals are filled in order of their increasing energies. So, for this we need to know which orbital is of lower energy and which is of higher energy. This problem can be solved by learning the (n+l) rule, where n stands for principal quantum number and l stands for azimuthal quantum number.

Lower the value of (n+l), lower will be the energy of the orbital. If two orbitals have equal (n+l) values, then the orbital having lower n value is said to have lower energy associated with it. So, following the Aufbau principle and (n+l) rule, we can arrange the orbitals in order of increasing energies as:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, and so on.
From this series, it is clearly visible that 1s being the lowest energy level, will be filled first. Moreover, there is no energy level having lesser energy than 1s.
So, on absorbing radiation of sufficient energy, electrons will be transferred from ground state, i.e. 1s state to higher levels, such as 2s or 3s. And in order to emit the absorbed radiation, the electrons need to go back to the lower energy levels. Since, 1s is the lowest energy level, the electrons cannot go to any further lower energy level. Hence, emission from 1s is not possible.

So, the correct option is (a).

Note: In quantum mechanics, the principal quantum number (symbolized n) describes the electron's state. Its values are natural numbers (n = 1,2,3,4,5,6…...). The azimuthal quantum number tells the shape of the orbital.