The correct answer is 4: (a) Sir Philip Sidney and (b) P.B. Shelley, because both of these writers are known for their famous essays titled "A Defence of Poetry."
Sir Philip Sidney: He wrote "An Apology for Poetry" (also known as "A Defence of Poetry") in 1595. It is one of the first works in English literary criticism, where Sidney defends the value of poetry against its critics.
P.B. Shelley: He wrote "A Defence of Poetry" in 1821, which is a seminal essay arguing for the importance of poetry as a civilizing and imaginative force in human culture.
The other options don't fit:
(c) Dr. Matthew Arnold: Arnold was a major poet and critic, but he didn't write an essay specifically titled "Defence of Poetry." His famous critical work is "The Study of Poetry."
(d) T.S. Eliot: Eliot wrote extensively on poetry and criticism, but not a piece called "Defence of Poetry." He is known for works like "Tradition and the Individual Talent."
Thus, (a) Sir Philip Sidney and (b) P.B. Shelley are the two correct answers.