Gravitational redshift is a complex phenomenon where electromagnetic radiation (light) is changed as a result of gravitational time dilation. This occurs when light is released from a source that is a gravitational field (i.e. a star) and observed away from the source at a higher gravitational potential (i.e. a planet). Due to the differences in gravity with respect to the light source, time passes faster to the observer and as frequency is the inverse of time, this reduces the frequency of the light observed, effectively 'red shifting' it or moving the visible light towards the red end of the spectrum.
So the correct answers are:
A. Wavelength of light lengthens
C. Frequency of light decreases
D Observed on Earth appears more redlight
