Final answer:
The correct answer to the question "Are they working now?" is option (d) 'aren't', since it is the proper contraction of 'are not' for the plural subject 'they' in the present continuous tense.
Step-by-step explanation:
The question "Are they working now?" refers to a group of people and their current state of activity. To answer this question according to proper English grammar, we must use the correct auxiliary verb that matches the subject - which is 'they' - in the present continuous tense. The proper auxiliary verb to use for plural subjects in the present continuous tense is 'are'. Therefore, the correct answer to fill in the blank is option (d) aren't.
Remember, in negative sentences, 'are' becomes 'are not', which is often contracted to aren't in spoken and informal written English. For instance, if we were to say that they are not working right now, we would say "They aren't working now." This contraction makes the negative form of the present continuous tense for the pronoun 'they'.