A group of scientists are studying the effects of different fertilizers on the growth of pea plants. They began their study with 10 pea plants. They gave each plant a different type of fertilizer and tracked their growth over a period of 2 months. the result of this experiment weren't considered valid by other scientists because there was no control group. which of the following represents an appropriate control group for this experiment?

A control group of the experiment is the one in which no treatment or intervention is given. So, in your case, the pea plants in which was not subjected any of the fertilizers, will serve as a control group. This is a negative control. A positive control group is the one in which an established treatment is given, which definitely leads to the production of the desired character. So, in your case, a very good well-known fertilizer which has a positive control on the growth of the pea plant, would serve as a positive control.