The correct answer is: A. They were laughing at the clown.
The sentence “The clown was being laughed at by them” is in the passive voice. In the passive voice, the subject of the sentence is acted upon by the verb. In this case, the clown is the subject and is being acted upon by the verb “laughed at.” The verb “laughed at” is a transitive verb, which means that it requires a direct object. The direct object of the verb “laughed at” is “them.” Therefore, the sentence “The clown was being laughed at by them” means that “them” were laughing at the clown.
The other options are incorrect because they do not make sense in the context of the sentence. Option B, “They were laughing on the clown,” is incorrect because the preposition “on” is not used with the verb “laugh.” Option C, “They laughed at the clown,” is incorrect because it is in the active voice, not the passive voice. Option D, “The clown was laughed at by them,” is incorrect because it is the same as option A, but with the word order reversed.