Answer
Graph c.
Work Step by Step
Recall $\Delta V = -\vec{E}\vec{s}\qquad \rightarrow \qquad \displaystyle \vec{E} = \frac{\Delta V}{\vec{s}}$ where $\vec{s}$ is the position vector.
Graph $c.$ is correct because there is a uniform electric field (all the vectors have the same length) pointing in the upward direction. Since $\vec{E}$ is uniform for all $y$, that means that $\Delta V$ grows at a rate proportional to $y$ so that $\vec{E}$ stays constant. The only graph with a negative and constant slope is $c.$
