Answer
An electron has a longer de Broglie wavelength than a proton when they have the same kinetic energy.
Work Step by Step
The de Broglie wavelength is given by $\lambda=\frac{h}{p}$.
If the electron and proton have the same kinetic energy KE, the smaller mass electron has the smaller momentum $p=\sqrt{(2)(m)(KE)}$.
An electron has a longer de Broglie wavelength than a proton when they have the same kinetic energy.
