Final answer:
Coal forms from plant material that accumulates in a(n) reducing depositional environment
Step-by-step explanation:
Coal forms from plant material that accumulates in a reducing depositional environment. This environment facilitates the preservation of plant material because it prevents the decomposition that would normally occur if oxygen was present.
Over time, high temperatures and pressures transform this material into coal, making the correct answer to the student's question 'a) plant material; reducing.' Coal is formed from plant material in a reducing environment where decomposition is inhibited, leading to the gradual transformation into coal under high pressure and temperatures.