1. He isn't watering the plants, ... ?

The correct answer is b) isn't he.

Explanation: The sentence is in the present continuous tense. When forming a question from a statement in this tense, we invert the subject and the auxiliary verb. So, 'He isn't watering the plants' becomes 'Isn't he watering the plants?'. However, since the original statement is negative, and the question is asking for confirmation or a tag question, the correct form is to use the same auxiliary verb and pronoun. Given the options, 'isn't he' is the most grammatically appropriate completion to form a question about the subject's action.