Which of the following is an indicator of a child's language ability?

A child's language ability is often assessed by the vocabulary they possess, as vocabulary is a fundamental component of language development. Knowing a wide range of words demonstrates not just the ability to communicate but also an understanding of language structure and meaning. A robust vocabulary allows children to express their thoughts and feelings more clearly and to engage effectively in conversations.

While other factors like following instructions can indicate comprehension skills, counting refers more to numerical understanding than language ability. Similarly, the number of books at home can support literacy but does not directly measure a child's language skills. Therefore, the number of words a child knows is a direct indicator of their language ability, reflecting both their capacity for communication and their linguistic development.