Concept:The focus is on identifying the metrical foot used in the given line of poetry.
Explanation:The line to analyse is: "When you are old and grey and full of sleep."
Begin by breaking the line into natural sound units or feet.
The rhythm falls as: "when YOU | are OLD | and GREY | and FULL | of SLEEP."
In each unit, a lightly stressed syllable is followed by a strongly stressed syllable.
An unstressed syllable followed by a stressed syllable is called an iamb.
Since the line contains five such iambs in a row, the metre is iambic pentameter.
Therefore, the correct option is iambic.
Answer:D. iambic