Final answer:
A child learning how different parts of speech form sentences is learning about syntax, how words are arranged to create meaningful sentences. Hence, option (a) is correct.
Step-by-step explanation:
When a child learns about how nouns, verbs, adjectives, and prepositions go together to form sentences, the child is learning about syntax. Syntax is how words are organized into sentences, which is a critical aspect of language structure and grammar. It contrasts with semantics, which involves the meaning derived from words and sentences, morphology, the study of the structure of words, and phonology, the study of the sound systems of languages.
The syntax is vital as it dictates how the individual parts of speech must be placed to convey the correct meaning. Without a proper understanding of syntax, sentences could be jumbled and difficult to understand. Mastering this aspect of language allows for effective communication and comprehension of both simple and complex concepts.