Answer:
d. a single variable.
Step-by-step explanation:
A controlled experiment is one in which the researcher can can keep all other factors/variables uniform and see the effects of a particular factor/variable on certain subjects.
For example, in order to determine the effects of different light intensities on the growth of a plant species. All the growth conditions will be uniform for all the subjects (the plant species) except the light intensity. Any discrepancy in growth can thus be attributed to the different light intensities.
The light intensity is varied while other factors are kept uniform.