The hydrogen-like species $Li^{2+}$ is in a spherically symmetric state $S_1$ with one radial node. Upon absorbing light the ion undergoes transition to a state $S_2$ has one radial node and its energy is equal to the ground state energy of hydrogen atom. The state $S_1$ is

Question

$(a)\;1s\qquad(b)\;2s\qquad(c)\;2p\qquad(d)\;3s$