Answer:
Experiment
Step-by-step explanation:
For such an investigation an Experiment would be the better option. This is because in an experiment you would have both an experimental group and a control group. This would allow you to use fertilizer on one but not the other, therefore, providing seperate results that you can use to compare with one another. This ultimately helps you determine the actual effects that fertilizer has on the growth of plants, since the fertilizer would be the only variable in the experiment. An observational study would only allow you to observe the plants in a natural scenario, without interfering and seperating the plants into different groups with different environments.