According to Einstein’s photoelectric equation:

The correct answer is Option (1) → \( K_{\max} = h\nu - \phi_0 \)

Einstein’s equation is energy conservation for the absorption of one photon by one electron. The photon energy \( h\nu \) first pays the work function \( \phi_0 \) needed to leave the metal, and what is left over is the maximum kinetic energy of the photoelectron.

    \( h\nu = \phi_0 + K_{\max} \Rightarrow K_{\max} = h\nu - \phi_0 \)

NCERT (2026-27):
Einstein’s Photoelectric Equation: Energy Quantum of Radiation – “… In photoelectric effect, an electron absorbs a quantum of energy (hν) of radiation. If this quantum of energy absorbed exceeds the minimum energy needed for the electron to escape from the metal surface (work function \( \phi_0 \)), the electron is emitted with maximum kinetic energy

    \( \displaystyle K_{max} = h\nu - \phi_0 \qquad (11.2) \)

… Equation (11.2) is known as Einstein’s photoelectric equation.”