The correct option is
A to dance
In order to fill in the blank with the most suitable verb, we need to understand the usage of 'make'. 'Make' means to force someone to do something. If 'make' is used in a passive (voice) sentence, we use the word 'to' along with the verb. In an active sentence, we do not use 'to' with the verb. The verbs without the marker 'to', are called bare infinitives.As the given sentence is in the passive voice, we will not use the bare infinitive here; we will use the verb with the marker 'to'.
NOTE: Here, 'made' is used as the main verb, which describes the sentence in past tense. Though the sentence is in past tense, the verb with the marker 'to' will always be in its original form.
Option A - 'Dance' is a verb in its regular form, which is used in sentences with active voice. As the given sentence is in the passive voice, we must use the infinitive with the marker 'to'. Hence, option A is incorrect.
Option C - 'Dances' is the third person form of the verb 'dance'. As the infinitive is not in its original form and also, it isn't accompanied by the marker 'to', we can't use 'dances' here. Hence, option C is incorrect.
Option D - 'Dancing' is the participle form of the verb 'dance'. As explained above, we need the infinitive with 'to'. Hence, option D is incorrect.
Option B - 'To dance' is absolutely correct as the infinitive 'dance' is in its original form and is accompanied by the marker 'to'. Hence, option B is the correct answer.
'The clown is often made to dance by the children.'