Who among the following illustrated the works of Thomas
Gray and Robert Blair?

The correct option is William Blake.

William Blake (1757–1827) was a prominent English poet, painter, and printmaker of the Romantic Era. Besides creating his own illuminated books, he was highly sought after to illustrate the works of other classical and contemporary writers, including Thomas Gray and Robert Blair.

Illustration of Thomas Gray's Works:
In 1797, Blake was commissioned by his friend and sculptor John Flaxman to illustrate the poems of Thomas Gray. Blake produced a series of 116 large watercolour illustrations built around the printed text of Gray's poems, including famous works like Ode on a Distant Prospect of Eton College and Elegy Written in a Country Churchyard.

Illustration of Robert Blair's Works:
In 1805, the publisher Robert Cromek commissioned Blake to create illustrations for Robert Blair's celebrated poem, The Grave. Blake designed a series of powerful and visionary drawings depicting death, the soul, and resurrection, which were etched by Luigi Schiavonetti and published in 1808, becoming some of Blake's most widely known illustrations during his lifetime.