a
\(\Rightarrow \mathrm{F}_{\mathrm{net}}=2 \mathrm{F} \cos \theta\)

\(F_{n e t}=\frac{2 k q\left(\frac{q}{2}\right)}{(\sqrt{y^{2}+a^{2}})^{2}} \cdot \frac{y}{\sqrt{y^{2}+a^{2}}}\)

\(F_{n e t}=\frac{2 k q\left(\frac{q}{2}\right) y}{\left(y^{2}+a^{2}\right)^{3 / 2}} \Rightarrow \frac{k q^{2} y}{a^{3}}\)

So, \(F \propto y\)