n=5ton=4
n=6ton=1
n=6ton=5
n=5ton=3
Energy levels are inversely proportional to the square of n. Therefore, the least energetic photon would be when the difference between the energy levels is least. This would be when the difference is only 1 level of n. Higher the n, lower the energy. Thus, the transition from n=6 to n=5 would produce the least energetic photon.