Answer:
Option A, control
Step-by-step explanation:
A control group in an experiment is a group which is not exposed to the variables used in the experiment so that the experiment group observations can be compared with the observations of control group and a difference in both the group can be established.
Here , mold spores receiving no ultraviolet light will act as a control group which is not exposed to the variable "light i.e ultraviolet light".
hence, option A is correct.