1) The scenario is a controlled experiment because there were controls and a single experimental group in order to find correlations in data. The manipulated variable would be the fertilizer because it is the only factor different between Group A and Group B.
2) A one-time trial is not sufficient enough in terms of evidence to support a theory. Theories have numerous repetitions and replications to prove the correlation to be strong.
3) (The graph does not appear, but I can assume there are labels on the axes.) The labels on the graph's axes show that the y-axis is the plant growth over time.