She ... have watered the garden as I had already done it.

The correct answer is "needn't".

Answer: needn't

Explanation: The sentence means that she didn't need to water the garden because I had already done it. Therefore, the correct modal verb is "needn't", which expresses the lack of necessity. The other options, "can't" (cannot) and "mustn't" (must not), do not fit the context of the sentence.