This is where the mentally handicapped child simply repeats what is said to him, like a parrot.
How might we explain this symptom? One possibility is to say something like "It's because he or she is autistic". This silly non-explanation is in fact often used by care staff working with s.l.d. children, but is obviously of no value whatsoever.
This verbal behaviour is the equivalent of the imitate or copy response to the demonstration of kicking a ball in the DDST. Here the response is required. It is nevertheless a very primitive response, and echolalia is not the required type of response, except in a memory for sentences type of test, as in the Stanford-Binet intelligence test.

What is usually wanted is a meaningful response, e.g.
Qu. "What is the opposite of up?"
Answ. "Down"

We can use the system used in the early psychological experiments, that using the concepts of
S, the stimulus,
T, the task, Aufgabe,or Determining Tendency, (DT)
R, the response.

The mentally handicapped child, because he is mentally handicapped, and of low IQ, is unable to respond to the verbal stimulus, in a manner proper to this very high level skill.
In a similar way such behaviour will also be found in other individuals functioning at a relatively low mental level, e.g. normal infants. The phenomenon in not some mysterious attribute, especially of 'autistic' individuals, it is a simple consequence of lower intellectual ability.
(See other 'mysterious' symptoms of 'autistic' children such as mis-use of, or difficulty with, personal pronouns. In the latter case "I", "me", "you", "mine", "yours" etc can refer to, or mean quite different people, depending on who is uttering the word, while a proper noun such as Samantha Jones, as far as the child is concerned only means Samantha Jones.)
If we handicap a normal adult person appropriately, he might do the same. For example a person confronted with someone speaking a foreign language he he does not understand, or understands only slightly, might do this. This might be partly to give him a chance to think of a response, but is offered to show the other person that he is at least acknowledging that a communication, a meaningful message has been sent, that he has heard it correctly, and is working on an answer.
The scenario of encountering an alien species might involve this idea.
Note that there is always the initial problem, in any behaviour, of deciding if a response is communicative or not. I recall, in a signing class years ago, having to consciously reject a gesture made by our sign language teacher as a sign, when she made to scratch her nose!