Can someone me with this one?

you!

The correct answer is D. changes in how much of the sunlit side of the moon faces earth. 

Here's why: 

The shadow of earth falls upon the moon causing phases. ex. full moon, half moon, etc.  When the moon passes in front of the sun what will result is a new moon. However, when the sun is behind the earth, it will illuminate the moon and will result in a full moon. 

Knowing this, we can easily conclude that changes in the area of the sun over a period of time will result in different moon phases. (Remember, it all depends on the sun.) 

Themes; subfields is the answer