The increase in land area which

The increase in land area which the virus has colonized suggest that birds are the more usual host for the virus, strengthening researchers’ original contention that migratory birds have contributed to the virus's spread across the nation.

A.  suggest that birds are the more usual host for the virus,
B.  suggest that birds are the more usual host to the virus,
C.  suggests that birds are the more usual host for the virus,
D.  suggests that birds are the more usually host for the virus,
E.  suggests that birds are the more usually host of the virus,

The best answer is C.  In choices A and B, the verb suggest does not agree with its singular subject, rise.  Choices D and E use the adverb usually where the adjective form is required.