Select the option that can be used as a one-word substitute for the given…

Concept

A one-word substitution condenses a phrase into one word without losing its central meaning.

For an emotion word, identify both the kind of feeling and the object or time to which that feeling is directed.

Application

The phrase describes a longing directed toward something personally known in the past. Nostalgia is the noun for a wistful or sentimental longing for a past time, condition, or experience.

Contrast

• Nostalgia denotes sentimental longing for a remembered past.

• Unhappiness denotes a general state of sadness or dissatisfaction.

• Sentiment denotes a feeling, attitude, or opinion without necessarily referring to the past.

• Retrospection denotes the act of looking back on or reviewing past events.

Cross-check

The defining features are longing plus a personally remembered past; together they point specifically to nostalgia, not merely sadness, a general feeling, or the act of reviewing the past.

Therefore, the required one-word substitute is nostalgia.