The correct answer is A. Experimental group
Step-by-step explanation:
In experiments, the experimental group refers to the group of subjects that receives special treatment or in which the scientist or researcher intervenes. This is the opposite of a control group that does not receive treatment and therefore, it is used as a reference to measure the effect of the treatment.
This implies, in the experiment presented Group A is the control group as they do not have special treatment because they sleep normally and Group B is the experimental group as the researcher modifies their sleep time and reduces it two hours which means in this group there is a treatment or intervention.