(b) fish, heterotroph
A heterotroph is an organism that is unable to synthesize its own food, and therefore, has to rely on other sources, specifically plant and animal matter.
(c) mosquito, parasite
Parasites are an incredibly varied group of organisms that live within host cells. They are smaller than their host organism and reproduce faster by causing more damage to the host.
(d) mushroom, saprophyte
Saprophytes are the living organisms that live and feed on dead and decaying organisms. They are considered extremely important in soil biology.