The line "Heard melodies are sweet but those unheard are sweeter" is taken from John Keats' famous poem "Ode on a Grecian Urn". This quote reflects the theme of the poem, which explores the idea of eternal beauty and the contrast between the real world and the world captured in art. Keats uses the concept of an urn, which is still and silent, to symbolize an idealized form of existence that is free from the ravages of time. The line suggests that things that are imagined or not yet experienced often hold a greater appeal than those that are already tangible or real.

Here’s a breakdown of the options:

• Robert Frost is known for works such as "The Road Not Taken" and "Mending Wall," but he did not write this line.

• William Wordsworth is famous for works like "I Wandered Lonely as a Cloud" and "Tintern Abbey," and while his poetry often celebrates nature, he is not the author of this line.

• John Keats, the correct answer, is a Romantic poet known for his exploration of beauty, art, and transcendence. This particular line is from his poem "Ode on a Grecian Urn," written in 1819. Keats’ work frequently examines the tension between the fleeting nature of life and the permanence of art, with this quote illustrating that concept.

• PB Shelley is another Romantic poet known for works like "Ozymandias" and "To a Skylark," but he did not write this line.

John Keats' "Ode on a Grecian Urn" continues to resonate with readers for its meditations on immortality, beauty, and the contrast between the static and dynamic elements of life.