The correct answer is: C. are bred
The verb “breed” means to produce offspring, and it is used in the passive voice in this sentence. This means that the animals are not breeding themselves, but are being bred by humans. This is done for a variety of reasons, such as to maintain a healthy population of the species, to provide animals for research or education, or to simply provide entertainment for humans.
The other options are incorrect because they do not agree with the tense of the sentence. Option A, “have breed,” is in the past tense, but the sentence is about something that is happening in the present. Option B, “are being breeding,” is in the present progressive tense, but the sentence is about something that is happening habitually or regularly. Option D, “breeds,” is in the simple present tense, but the sentence is about something that is happening to the animals, not something that they are doing themselves.