We note that $7=\sqrt{49}$ and that $\sqrt{40}<\sqrt{49}<\sqrt{50}<\sqrt{60}<\sqrt{70}<\sqrt{80}$.
This means that $\sqrt{40}$ or $\sqrt{50}$ is the closest to 7 of the given choices.
Since $\sqrt{40} \approx 6.32$ and $\sqrt{50} \approx 7.07$, then $\sqrt{50}$ is closest to 7.