Which diagram represents electromagnetic energy with the shortest wavelength?

The correct answer is option B. 

The wavelength is related to frequency as c = νλ
where, ν is the frequency
λ is the wavelength
c is the velocity of the electromagnetic wave.

Now, wavelength is inversely proportional to the frequency

High-frequency ⇒ low wavelength

Now, from the diagram, highest frequency is in figure B. Here, in this figure, more number of waves occur in the given time., see figure B gas the highest frequency. So, it will have the shortest wavelength.