Final answer:
Reversing the polarity in Galvani's experiment still results in muscle contraction due to electrical current triggering nerve impulses. The correct options are A and D respectively.
Step-by-step explanation:
If the polarity is reversed in Galvani's frog leg experiment, muscle contraction still occurs because it is the electrical current, regardless of the direction of the polarity, that causes the muscles to contract. Therefore, the correct answer to the first part of the question is a. Muscle contraction still occurs.
What is responsible for muscle contraction in the frog leg experiment is the external stimulus provided by the electrical current, which triggers the nerve impulses that lead to muscle contraction. Hence, the right answer to the second part of the question is d. Nerve impulses.