An action in a sentence can be represented in two ways, namely active and passive voice.

The passive voice always uses the past participle form of the main verb irrespective of any tense. Only the auxiliary verbs depend upon the sentence given in active voice.

The correct answer is B)A perfect dam has been built by the beavers across the stream.

The option B is correct as the given sentence is in the active voice and to transform it into the passive active voice, the subject of the given sentence (the beavers) comes in the place of the object (a perfect dam) and vice versa. Also, as the sentence is in the present perfect tense, the past participle form of the main verb (build) is used after 'been'.

Options A, C, and D are wrong as the given sentence is not in the past perfect tense, the helping verb 'have' cannot be used in the transformed sentence as its subject is singular, and the sentence is not in the past continuous tense respectively.