From the figure describing photoelectric effect we may infer...

According to given figure, the graph between stopping potential and frequency is a straight line, therefore stopping potential $\left(V_{0}\right)$ and hence maximum kinetic energy $\left(K_{\max }=e V_{0}\right)$ of photoelectrons depends linearly on the frequency. Since, threshold frequency for $\mathrm{Na}$ is less than that of for Al, therefore work function for Na will be lesser than that of Al. Hence, $\mathrm{Na}$ is better photosensitive material than $\mathrm{Al}$.