In order to find out the correct form of the verb needed, let's first understand these tenses. Present continuous tense is used to express an action that is continuing at the moment. Usually, in this case, the start time of the action isn't specified. Present perfect continuous tense is used to express an action that has started in the past, continuing in the present, and might go on or end in future. Here, usually, the start or the end time of the action is given. This sentence says - 'are about to finish', which means that they started in the past, and will finish in a while. Hence, we will use the perfect continuous here. Option D - 'Have been reciting' is the correct answer. 'They have been reciting the mantras for art hour and are about to finish.' Options A, B and C are incorrect as they aren't in the perfect continuous tense.