4. what does einstein's theory of relativity explain? explain

a. how magnetism reverses the fields of the poles
b. how light interacts with moisture
c. how gravity interacts with planets
d. how light interacts with gravity

General Relativity is based on the equivalence principle, which expresses the following:

A gravitational force and an acceleration in the opposite direction are equivalent, both have indistinguishable effects.

In other words, for general relativity, gravitational force and acceleration in the opposite direction have the same effect.

Then, in the case of light:

In the absence of accelerations photons travel in a straight line, but if we move quickly (accelerate), we will see how the trajectory of the photon curves.

But, how is this possible is photons has no mass and cannot be affected by gravity?

Because of the equivalence principle.

This effect is more evident in distant galaxies, which can act as gravitational lenses. So, in this way the rays of light are deflected if there is a mass accumulation nearby.

a. i. only is correct.