well.
First of all let me say that the question for me is good: at the first time it seems easy but indeed you need some brain power to disentagle.
Some scientists suggest the moon had been formed out of part of the Earth, which was dislodged perhaps by a meteor.
A) the moon had been formed out of part of the Earth, which was dislodged perhaps
X suggest ---> as soon as you see this, the question is : what ?? what they say. I immediatly think to THAT: we need a restrictive modifier because
a priori the meaning of the sentence is: a part of something drifted away from this, going to form another corpus (the moon). That say, we need a resticted modifier to convey the scope of a general noun to a relevant subset. Basically, the other options also communicate that the entire earth took part in this process. And this is wrong: we have ONLY one part comes into the picture.
B) that the moon was formed from part of the Earth that had perhaps been dislodged
here we have the use of THAT correctly used. The sentence is parallel and the events, in their unfolding, are pretty much clear.
CORRECTC) that part of the Earth formed the moon, which was dislodged perhaps
her we have THAT + WHICH: thi is the worst scenario we could have. really far from home
D) the moon was formed out of part of the Earth, having perhaps been dislodged
same as A does + having (remember in general the use of having is really really bad on the gmat)
E) that the moon had been formed from part of the Earth, which perhaps had been dislodged
sama as C
This could comes in handy
https://magoosh.com/gmat/2012/that-vs-which-on-the-gmat/Hope this helps