Answer:
E
Step-by-step explanation:
The answer is E because the size of the cage will affect the mice.
The answer cannot be A, because keeping the mice in solitude in separate rooms would have made no difference to the experiment because in either situation, the mice are in isolation.
The answer cannot be B, because in giving the one group less food, this would change the behaviour of the mice completely. If the mice don't receive enough food and water, they may become lethargic which would affect the results.
The answer cannot be C, because E is the correct answer. If she had kept the cages uniform (and hence kept the conditions the same) then this answer would be right, however in this example, her experimental design is flawed.
The answer is not D, because if she were to do this, she would, once again, be introducing factors that may affect the results of the test.