Number of spectral lines possible when the electron de-excites from the nth energy level to the ground state isN=2(n2−n1)(n2−n1+1)Here the electron is in the 4th energy level, so n2=4 and the lowest level is n1=1.N=2(4−1)(4−1+1)=23×4=6These lines correspond to the transitions 4→3, 4→2, 4→1, 3→2, 3→1 and 2→1.Hence the number of spectral lines emitted is 6.So, the correct option is (a).