Step 1: Since the field between the plates is UNIFORM (constant magnitude), potential varies LINEARLY with distance (since $E=-dV/dx=$constant means $V$ is a linear function of $x$).

Step 2: $V = V_0+\alpha x$, where $\alpha$ is a constant (positive or negative depending on field direction) — this is a linear (first-degree) relationship, not quadratic or fractional powers.

Step 3: Answer: (A) $V=V_0+\alpha x$.