CBSE Class 12 · 2022 · Modern PhysicsShort Answer

Each electron is ejected by absorbing a single photon of energy $h\nu$. Emission needs this energy to exceed the work function, i.e. $h\nu \ge {\varphi }_0$ or $\nu \ge {\nu }_0$.
Increasing the intensity only increases the number of photons, not the energy per photon. If $\nu <{\nu }_0$, no single photon can free an electron, however many photons strike. Hence it is the frequency, not the intensity, that decides whether emission occurs.