Answer
$\vec{F}_a = \vec{F}_b = \vec{F}_c = \vec{F}_d = \vec{F}_e$
Work Step by Step
Because the electric field is uniform between the plates of a capacitor, the electric field vector is $\vec{E} = \displaystyle \frac{\eta}{\epsilon_0}$.
The electric force on a proton inside the capacitor is $\vec{F}_{electric} = q\vec{E} = q\displaystyle\frac{\eta}{\epsilon_0}$. Here we see that the distance from the proton to either plate is not in the equation, and therefore irrelevant to this problem.
Therefore, no matter where the proton is in the capacitor, it will experience the same force everywhere.
$\vec{F}_a = \vec{F}_b = \vec{F}_c = \vec{F}_d = \vec{F}_e$
