We have been looking for a new flat ______ ages.

Answer: 'for'

Explanation: The sentence "We have been looking for a new flat for ages." uses the present perfect continuous to describe an action that began in the past and is still continuing. "For" indicates the duration of that action (an amount of time).

• Why "since" is incorrect: "since" requires a specific starting point in time (for example, since 2010 or since last week), not a vague duration like "ages."

• Why "during" is incorrect: "during" refers to an event or period (for example, during the summer) and does not naturally express a continuous duration with "ages."

• Why "in" is incorrect: "in" indicates when something happens (for example, in July) or location; it does not express a duration like "for ages."

Tip: Use "for" with durations (for two hours, for months, for ages) and "since" with a specific start point (since 2018, since Monday).