Answer:
B) Experiment: you imposed a treatment (new fertilizer on some plants, old on others) to determine its effect.
Step-by-step explanation:
In this situation, the fertilizer developer wants to evaluate a hypothesis, not just observe, by subjecting plants to two different treatments (new and old fertilizer) in order to determine which treatment provides a more desirable effect (more plant growth).
This process configures an experiment.
The answer is alternative B)