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