A one arm is lost it quickly replaces it, sometimes by the animal overcompensating and--- it seems as if the plural starfish is pronounced by the first, singular it; in addition in an active voice sentence, the use of by is improper

B one arm is lost it is quickly replaced, with the animal sometimes overcompensating and ------- seems ok with the SV problem avoided altogether. Here the it should logically refer to the arm. correct choice

C they lose one arm they quickly replace it, sometimes by the animal overcompensating, -----use of by is improper in an active voice setting. It should be with the animal rather than by the animal; overcompensating, growing is improper co-ordination. There should be an and in between

D they lose one arm they are quickly replaced, with the animal sometimes overcompensating,’---use of they means that the starfish themselves are replaced; over compensating should be followed by and

E they lose one arm it is quickly replaced, sometimes with the animal overcompensating, --- in a passive voice we need to use by rather than with ; in addition overcompensating should be followed by and