The correct option is C (b)
Let’s examine the sentence: “There is no reason why he should not come since he is in town and is free.” The sentence requires a word or phrase to connect the clause “there is no reason” to the clause “why he should not come.” The compound preposition “as to” placed before “why” can bring about this connection. Compound prepositions are generally created by prefixing a preposition to a noun, adjective or adverb. They establish relationships between two nouns or clauses. The correct sentence here would be, “There is no reason as to why he should not come since he is in town and is free.” So, the error is in Part b of the sentence. Hence, Option B is the correct answer.