In which poem of Wordsworth, does the line, "Our birth is but a sleep and a forgetting", occur?

Option 4 : Immortality Ode

The correct answer is Immortality Ode.

•  The full title of the poem is "Ode: Intimations of Immortality from Recollections of Early Childhood".
• The poem was completed in two parts: with the first part comprising of four stanzas about the innocence of childhood. When Wordsworth gave the first part of the poem to Coleridge for his perusal, Coleridge responded with a poem of his own, named "Dejection: An Ode". 
• The full poem was finished in 1804 and published in a book named Poems, in Two Volumes in 1807.

•  The fourth stanza of the ode ends with a query, and Wordsworth answers it with seven additional stanzas completed in early 1804.
• It is an irregular Pindaric ode where Wordsworth introduces readers to the idea of pre-existence, the idea that the soul existed before the body, and will continue to exist afterwards. 
• In the given line, Wordsworth is attempting to suggest that our earthly existence is just a fraction of our immortal selves.
• Modern critics have referred to this poem as a "Great Ode" and have ranked it among Wordsworth's best.