P.B. Shelley’s Adonais laments the death of:

The correct answer is John Keats.

Adonais: An Elegy on the Death of John Keats, Author of Endymion, Hyperion, etc. is a pastoral elegy written by Percy Bysshe Shelley in 1821.
It is widely considered to be one of Shelley’s masterpiece poems and one of the greatest elegies in English literature.

John Keats, a fellow English Romantic poet, died in Rome of tuberculosis in February 1821 at the young age of 25.
Shelley, who admired Keats's genius, was deeply affected by his untimely death. He also believed that the severe and hostile reviews Keats's work received in prominent journals, such as the Quarterly Review, had broken Keats's spirit and hastened his death.

The title of the poem, "Adonais," is a blend of the Greek name "Adonis" (the beautiful youth in classical mythology who was tragically killed by a wild boar) and the Hebrew name "Adonai" (meaning "Lord").
Through this pastoral elegy, Shelley mourns Keats by casting him as the fallen youth Adonis and criticizes the harsh critics who he felt contributed to the poet's tragic end.