The diagram shows an electromagnet made with copper wire, a steel nail, and a 1.5 V battery. Which change would make this electromagnet weaker?

A. Reverse the direction of the battery

B. Rap more coils around the nail

C. Replace the steel nail with a wooden stick

D. Replace the battery with a 3V battery

Correct answer is C

you'd use the equation that establishes a relationship between  energy  and  wavelength, which looks like this

e=h⋅cλ, where

λ  - the wavelength of the photon; c  - the speed of light, usually given as  3⋅108m/s; h  -  planck's constant, equal to  6.626⋅10−34j s

plug the value you were given for the energy of the photon in the above equation to get the wavelength

e=h⋅cλ⇒λ=h⋅ce

λ=6.626⋅10−34js⋅3⋅108ms−1.257⋅10−24j=1.581⋅10−1m