What is a testable explanation of or answer to a scientific question that is based on prior knowledge or research?

The correct answer is hypothesis.

Explanation:

An explanation, which is proposed, and which is accepted temporarily in order to look for an observed phenomenon is considered as a hypothesis. At certain occasions, it is also illustrated as an educated guess.  

A hypothesis is an initial step, which is taken by the scientists at the time of the scientific investigation and it functions as the ground on which the scientific investigation is reliant upon. A hypothesis needs to be testable, and the illustration provided about it must be based on previous scientific knowledge.  

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.