Answer
The electron has the longer wavelength.
Work Step by Step
The two particles, being accelerated from rest through voltages of equal magnitude, have the same kinetic energy.
The proton has a larger mass than the electron and (if they have the same KE) will have the larger momentum because $p=\sqrt{2 m KE}$.
The wavelength is inversely proportional to the particle’s momentum, $\lambda = \frac{h}{p}$, so the proton has the shorter wavelength.
