Correct option is B
Both (A) and (R) are true and (R) is the correct explanation of (A).
Assertion (A) is correct because children in their early childhood stage do use more verbs than nouns and ignore grammar rules. This is because they are still in the process of learning language and have not yet mastered the complexities of grammar.
Reason (R) is also correct because children in their early childhood stage do lack vocabulary range. This is why they use more verbs than nouns, because verbs are more concrete and easier to learn than nouns.