15. In many cases, they are poets and novelists, not historians, who catch the spirit of an era

Answer: D

Explanation:

It is poets and novelists, not historians, who catch the spirit of an era.

The phrase "it is" serves as a formal introductory clause that emphasizes the subject, highlighting the role of poets and novelists in capturing the essence of a time period.

A) they are

The phrase "they are" lacks the necessary introductory emphasis and does not adequately connect the subject with the action of capturing the spirit of an era. It is a less formal construction that does not fulfill the syntactical requirements of the sentence.

B) those

Using "those" would imply a reference to a previously mentioned group, which is not suitable in this context. The sentence requires a construction that introduces the poets and novelists as the subject, making "those" ineffective here.

C) that is

The phrase "that is" introduces a definition or clarification, which is not the intended function in this context. The sentence aims to assert a point rather than clarify, rendering "that is" inappropriate for the statement.

D) it is

"It is" effectively introduces the subject and emphasizes the importance of poets and novelists in capturing the spirit of an era. This construction is grammatically correct and aligns perfectly with the intended meaning of the sentence.

Conclusion

The correct answer, "it is," aptly emphasizes the role of poets and novelists, distinguishing them from historians in understanding the essence of a particular era. In contrast, the other options fail to provide the necessary structural and contextual relevance, thereby demonstrating why they are not suitable choices.