[AMU 2010] When the electron of a hydrogen atom jumps from n = 4 to n = 1 state, the number of spectral lines emitted is

Number of spectral lines = n(n−1)/2. When electron falls from n=4: possible transitions are 4→3, 4→2, 4→1, 3→2, 3→1, 2→1. Total = 4×3/2 = 6.