A thin semi-circular ring of radius r has a positive charge q distributed uniformly over it.

$\frac{\mathrm{q}}{2 \pi^{2} \varepsilon_{0} \mathrm{r}^{2}} \hat{j}$

$\frac{q}{4 \pi^{2} \varepsilon_{0} r^{2}} \hat{j}$

$-\frac{\mathrm{q}}{4 \pi^{2} \varepsilon_{0} \mathrm{r}^{2}} \hat{\mathrm{j}}$

$-\frac{q}{2 \pi^{2} \varepsilon_{0} r^{2}} \hat{j}$