A. Fertilizer B is better than Fertilizer A.

This statement is incorrect. This may look true at first, but the word "better" is an opinion. It does not include the dependent variable, plant growth. Is there a better choice?

B. During a four-week period, the plants with Fertilizer B received more water than the plants with Fertilizer A.

This statement is incorrect. The description of the experiment states that she gave each plant the same amount of water, so the water is a constant.

C. During a four-week period, the plants with Fertilizer A grew more than the plants with Fertilizer B.

This statement is incorrect. Look carefully at the data. Does the data support this conclusion?

D. During a four-week period, the plants with Fertilizer B grew more than the plants with Fertilizer A.