5.A.: Stationary states
The stationary states, or eigenstates, for a particle moving between two walls.
n is the quantum number that indexes the stationary states. E is the energy of a particle prepared in one of these eigenstates. The units of energy are chosen so that a particle in the ground state (n = 1) has energy equal to precisely 1 in those units. In these units, the energy of the eigenstates is equal to n2. You will learn that the energy of the hydrogen atom eigenstates also depends on a quantum number in a simple fashion.