The principle of the photoelectric effect is: a) the electromagnetic theory of light b) the special theory of relativity c) the principle of superposition d) the quantum theory of radiation

The electromagnetic theory of light.

A is correct .

Explanation:

Photoelectric effect :

When a light hits a metal then the electrons emits from the metal surface and  this process is called photoelectric effect. The emitted electrons are called photo-electrons.

According to this process, we conclude that some electrons were emitted by the zinc plate when exposed to ultraviolet light.

Electromagnetic theory of light :

The particles oscillate in perpendicular to the direction of travel of the wave motion.

Light is a electromagnetic wave that can be seen by the human eyes.

All electromagnetic waves can travel through a vacuum.

Hence,  The electromagnetic theory of light.

grav forces affect all objects magnetic forces affect magnetic objects and electric currents

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.