Final answer:
Peter Singer's theory of animal rights posits that all animals are equal in their capacity to suffer, which is the main basis for his argument on their moral consideration.
Step-by-step explanation:
Peter Singer's theory of animal rights maintains that all animals are equal in their capacity to suffer. Singer argues from a utilitarian perspective, suggesting that the ability to experience pain is a significant factor in considering the treatment of animals. Thus, the best representation of Singer's view in the provided options is (D) In their capacity to suffer. This does not mean that animals and humans are equal in cognitive abilities, intrinsic value, or in moral worth, but rather that their ability to suffer should be afforded equal consideration when making moral decisions.