Final answer:
In this experiment, the manipulated variable is the rabbit grass weeds, the responding variable is the rabbits, and the constants are the other environmental factors. Hence, A) is correct.
Step-by-step explanation:
The manipulated variable in this experiment is the Rabbit grass weeds, as it is being intentionally controlled and varied by the experimenter. The responding variable is the Rabbits, as it is the variable being affected by the change in the manipulated variable.
The constants in this experiment are the other environmental factors, which are kept constant throughout the experiment to isolate the effects of the manipulated variable.