Answer: C: Decomposers
Explanation: The mud and pond water likely contain some sort of decomposing bacteria. These bacteria can break down the complex nitrogen compounds in the dead fish, producing simple nitrogen compounds. These simple nitrogen compounds can then be used by the plants added to the terrarium.
A is wrong because producers (such as the green plants added to the terrarium) can not break down the complex nitrogen compounds in the fish's body.
B is wrong for a similar reason. Algae are primary producers. Because of this they also lack the ability to break down the complex nitrogen compounds found in the fish's body.
D is also wrong. A virus may be able to infect the fish and cause disease, but it lacks the ability to break down the complex nitrogen compounds found in the fish's dead body.
Since A, B, and D are wrong, along with the fact that we know that decomposers can break down the complex nitrogen compounds in the dead fish, we can conclude that C is the correct answer.