Answer:
A hypothesis
Explanation:
A hypothesis is an intelligent guess that is put forward in order to explain an observation. If it is observed that there is a difference in the rate of growth of plants that receives fertilizer and those that do not receive fertilizer, then we can hypothesize that;"If a plant receives fertilizer, then it will grow to be bigger than a plant that does not receive fertilizer". This is must now be subjected to a series of experiments in order to ascertain its validity.
This is the scientific method.