Final answer:
The strongest evidence for sophisticated animal language is the ability to engage in complex, symbolic communication with grammar and syntax. While animals don't match the open-ended nature of human language, primates show signs of a basic protolanguage ability.
Step-by-step explanation:
The strongest piece of evidence for the idea that animals are capable of developing and using a sophisticated language is D) Animals can engage in complex, symbolic communication with grammar and syntax. This type of communication goes beyond simple mimicry of sounds or the use of a limited set of symbols; it involves the use of structured linguistic systems that have rules similar to human language, although to a lesser degree of complexity. Our human languages allow for the combination of phonemes in an infinite number of ways to create new meanings, a trait that has proven to be a major biological advantage and critical to the development of human culture. While nonhuman primates, such as chimpanzees and bonobos, have shown the capability to use gestures or tokens in a rule-based manner, these systems are not as complex as human language and are often a product of human training.
True language involves the capacity to produce an infinite number of sentences out of a finite set of elements, and it seems that only humans and perhaps some species of primates and cetaceans show signs of this level of communication. Nevertheless, the evidence suggests that other great apes have the biological features that enable them to learn some aspects of human language, pointing to a basic capacity for protolanguage. Thus, the communication systems observed in animals, while impressive, typically fall short of the open-ended and creative nature of human language, though they do provide fascinating insights into the evolutionary origins of our linguistic abilities.