Energy of 1 photon = Eph = hc/l = (6.63X10-34 Js)(3.00X108 m/s)/550X10-9 m = 3.62X10-19 J

Power = 42 W = E/T = (N Eph)/t = N 3.62X10-19 J/3600 s

N = 4.2X1022 photons

Answer is E

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