Answer:
b) control
Step-by-step explanation:
An experiment, you control all variables. All except one variable should be the same between groups. The only variable that will change between groups if the one that you want to test the effect of. In experiments, you will need two or more groups; one of the groups should be the control. In the control group, the variable that you are going to test won't be present.
A control group is a group from which you will compare the results of the treatment group.
A variable is a factor; it could be dependent or independent. Dependent variables are those that you will measure, and they depend from independent variables. Independent variables are those that you will control.
Replication refers to the ability to reproduce the experiment. But in an experiment, you will need replicas, which are the number of plots (in this case) that you will use to calculate the effect.
A hypothesis is a possible explanation for the observation.