We used a formal method of study with control and experimental group to determine whether plants grew faster or slower when exposed to infrared light

The question is :
What is the student describing? 
A. using the scientific method 
b. proposing a hypothesis 
c.stating a solution 
d. making a conclusion

The correct answer is A.

The scientific method consists of proposing a hypothesis, testing the hypothesis and analyzing the results. The scientific method is based on verification or disproval of hypothesis. 
So, in the case of the plant experiments, the hypothesis was tested through experiments on two groups in a controlled environment.

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