What observation about light supported Einstein’s theory?

Explanation:

Do you mean the theory that space-time is defined by what's in it? And that mass warps space-time?

There are many instances where his observation turned out to be correct. The most popular one is the phenomena of gravitational lensing. We can see the light from stars that are actually behind the sun. The huge gravity of the sun bends space-time and this also bends light. We use gravitational lensing a the time when trying to see very distant stars or galaxies.

think about the oscillation of the toy shown in the video. when the toy is at the highest point of its motion, its velocity is zero and its acceleration is negative.

c)as speed changes, wavelength and frequency change.

everything changes!