The work function is the minimum amount of energy required to remove an electron from a metal.

Because the work function is the minimum energy required. Some electrons will need more than this minimum amount to escape the metal.

maximum possible value of kinetic energy of electron is 9eV

When light of suitable frequency falls on a metal surface, electrons are emitted from them. This effect is known as the photoelectric effect.

E=hf, h-plancks constant, f-frequency of light

We use electron volts because if we use Joules, the quantities will be very small.

the lowest frequency of electromagnetic radiation that will result in the emission of electrons from a specified metal surface.

Because the electron uses some energy to remove itself from the metal. KE=hf-Ф

It is the minimum potential difference required to stop the electron. (To remove all of its Kinetic energy)