Which of the following is the strongest piece of evidence for the idea that animals are capable of developing and using a sophisticated language?

(D) Apes have been able to use the words they know to express novel concepts.

One of the more impressive feats cited by those who believe that animals are capable of true language is that some apes have been able to use words they were taught to express novel concepts. For instance, Washoe, who didn’t know the word for refrigerator, was able to describe it as a “cold box.” Chomsky speaks of a language acquisition device, but the term does not refer to a physical structure in the brain. Even though primates are able to learn words that result in food rewards, critics of the idea that animals have language say these words are merely the result of operant conditioning and do not evidence true language. There is no doubt that animals including honeybees and dolphins can communicate with one another and with people to some extent; however, there is much debate over what should be considered “language.” Young apes are much slower to pick up new vocabulary than are young humans.