Question

Which line from “Ode to a Nightingale” by John Keats is written in iambic pentameter? A. And leaden-eyed despairs B. In the next valley-glades: C. Fade far away, dissolve, and quite forget D. Where but to think is to be full of sorrow,

Answer 1

The line from John Keats' poem that is written in iambic pentameter is C. Fade far away, dissolve, and quite forget.
Pentameter means that there are 5 meters in a line; one meter consists of 2 syllables, which means that a pentameter has 5 * 2 = 10 syllables in one line. The only option here that has 10 syllables is C. A has 6, B has 6, and D has 11. 

Answer 2

Which line from “Ode to a Nightingale” by John Keats is written in iambic pentameter it would be Fade far away, dissolve, and quite forget .

Hope this answer helps :) BTW the answer indeed is C