First of all "ITS " is a possessive pronoun that can correctly refer to possessive.
A. the freshwater snails that are the parasite's hosts for part of its life cycle
--> everything is CORRECT; ITS correctly refers to the possessive.
B. the freshwater snails that are the parasite's hosts in part of their
--> THEIR is wrong as per subject -verb agreement.
C. freshwater snails which becomes
the parasite's hosts for part of its life cycles
--. SV error
D. freshwater snails which become the hosts of the parasite during the parasite's life cycles
1) which is incorrectly used--> which introduces non essential part and also is used a comma.
2) unnecessarily wordy;
3) indicates "a parasite has several life cycles"
E. parasite's hosts, freshwater snails which become their
hosts during their
--> pronoun error
Hope this helps...!!
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