Schistosomiasis, a disease caused by a parasitic worm, is prevalent in hot, humid climates, and it has become more widespread as irrigation projects have enlarged the habitat of the freshwater snails that are the parasite's hosts for part of its life cycle.



A. the freshwater snails that are the parasite's hosts for part of its life cycle

The singular possessive pronoun “its” agrees with the singular possessive noun “parasite’s”

“That are” refers to”freshwater snails”

Correct


B. the freshwater snails that are the parasite's hosts in part of their life cycle 

The plural possessive pronoun “their” does not agree with the singular possessive noun “parasite’s”

Eliminate


C. freshwater snails which become the parasite's hosts for part of its life cycles

C can be eliminated because of the incorrect usage- “lifecycles”. The parasite does not have more than one lifecycle. 


D. freshwater snails which become the hosts of the parasite during the parasite's life cycles

Same as C.


E. parasite's hosts, freshwater snails which become their hosts during their life cycles

Same as B and C