2. Who ... the plants?

The correct answer is b) are watering.

Explanation: The question starts with "Who", which can refer to a singular person or multiple people. However, the verb form "are watering" suggests a plural subject or a question about the action happening to the plants in general. Since we are asking about the action of watering the plants, and the options include a continuous verb form, "are watering" fits best if the context implies multiple people watering or if we are asking about the ongoing action of watering. If it were a singular subject, 'is watering' would be expected, but that option is not available. 'Watering' alone is not a complete verb phrase for a question, and 'is watering' would be for a singular 'who'. Thus, 'are watering' is the best fit among the given choices for a general question about the action.