Harvester ants often strip a bush of all of its leaves. some people believe this the plant grow thicker, healthier stems. in an experiment, a student stripped off all the leaves from a set of plants. in a second set of identical plants, the student allowed ants to strip off the plants' leaves. in order to improve this experimental design, it is most important to add a set of plants

The correct answer will be- the control group of plants with leaves on them.

Explanation:

Harvester ants are the ants which collect the seeds of the plants and store them as a source of food in their nests in soil.

The experimenter observes that stripping of the plant leaves by the ant increases the thickness. The experiment was designed to study the thickness of the stem with one group in which the leaves were stripped by the humans and one group with leaves stripped by the ants.  

These cases represent the experimental groups in which stem thickness will differ but it lacked the control group which will have the plants with their leaves present on them and preventing them from stripping by human and ants.

This control group will provide a reference to the experimental groups and will improve the experiment.

Thus, the control group of plants with leaves on them is the correct answer.