Thinking is an internal and symbolic activity: a person manipulates ideas, images and concepts instead of handling the objects themselves. The psychological question, therefore, is what carries and organises those symbols inside the mind.

Words act as labels for concepts, so a richer word store lets a child draw finer distinctions between ideas and makes reasoning, planning and problem solving possible. Vygotsky held that inner speech grows out of social speech and becomes the chief tool of thought, which is why reasoning sharpens as a child's vocabulary widens.

The remaining choices do not fit. Putting an idea outward follows thinking rather than carrying it. Retention merely stores past experience and supplies raw material; it cannot by itself organise fresh reasoning. Since a genuine instrument of thought is already listed, rejecting every choice cannot stand either.

Language option is correct.