Answer:
C. all trials for fertilizer X and trials 1 and 4 for fertilizer Z
Step-by-step explanation:
Plants without fertilizer had a height of 7 cm or 8 cm. Taking these values as a reference, from the table we can see that fertilizer W has no effect, fertilizer Y has a negative effect (heights are below the reference), fertilizer X has a positive effect (heights are above the reference), and fertilizer Z has a positive effect in trials 1 and 4 and a negative effect in trials 2 and 3, that suggest this fertilizer is not as effective as fertilizer X, even though, it is concluded that it improved the growth rate of the plants given that it had a positive effect in some trials.