Why do we see a gravitational redshift in the spectrum of the sun?
a) spacetime curvature allows red photons of light to escape the sun more easily than blue photons, leading to an apparent redshift. b) because of gravity, the sun is always moving away from us, so we see a redshift. c) time runs slower on the sun than on earth, making lines in the solar spectrum have lower frequency and hence longer wavelength than normal. d) gravity makes light heavy, causing it to appear redder.

Time runs slower on the Sun than on Earth, making lines in the solar spectrum have lower frequency and hence longer wavelength than normal

Explanation:

When we talk about the visible electromagnetic spectrum, we know it starts in violet-blue and ends in red.  

In this context the astronomer Edwin Powell Hubble observed several celestial bodies, and when obtaining the spectra of distant galaxies he observed that the spectral lines were displaced towards the red one (red shift), whereas the nearby stars showed a spectrum displaced to the blue one.  

From there, it was deduced that the farther a body in the universe is, the more redshifted it is in its spectrum.

Now, the Sun is a massive star and the Earth is at certain distance from it, therefore time passes slower in the Sun than on Earth. This causes the spectrum of the Sun to present lines with a lower frequency or longer wavelength than normal, that is, more towards red.

there are no contact forces mentioned on the list of choices that you've provided.