In the relation $E_x=-\frac{\partial V}{\partial x}$, what does the symbol $\frac{\partial }{\partial x}$ instruct you to do?

Differentiate $V$ with respect to $x$ while treating all other variables as constants

Replace $x$ by a small change $\Delta x$ and compute an average slope only

Integrate $V$ with respect to $x$ while holding the electric field fixed

Differentiate $V$ with respect to every variable and then keep only the $x$ terms