To explain this question, let's clarify the relevant terms:

Duty time: A duty is essentially doing anything at all for the operator. You could be flying, in the sim, in ground school, doing office work, positioning etc.

Flight duty time: Starts when you report to the reporting point (normally in the ops room) and continues until a certain time after flight (typically 30 minutes after parking on stand is allowed for completing the tech log, etc).

‘Flight time’: the time between an aircraft first moving from its parking place for the purpose of taking off until it comes to rest on the designated parking position and all engines or propellers are shut down.

You need to know the flight time as it is the basis for determining the end of the flight duty and thus also the duty periods. Note: Flight duty time is an example of a duty time.

Looking at the incorrect answers:

Airborne time --> INCORRECT. This is the time from take-off until landing.

If a flight is to be considered a cross-country flight --> INCORRECT. Cross country flights are any flight beyond a certain distance from the departure airfield.

When the MEL cannot be applied --> INCORRECT. The MEL is intended to be used when parked on the ground and then the abnormal and emergency check lists are to used when the aircraft is in motion. However, the extent of the MEL states that it can be considered by the commander at any time to enhance situational awareness and safety decisions.

More information can be found here:

ICAO Annex 6
Chapter. Definitions

Duty. Any task that flight or cabin crew members are required by the operator to perform, including, for example, flight duty, administrative work, training, positioning and standby when it is likely to induce fatigue.

Flight time - aeroplanes. The total time from the moment an aeroplane first moves for the purpose of taking-off until the moment it finally comes to rest at the end of the flight.

Flight duty period. A period which commences when a flight or cabin crew member is required to report for duty that includes a flight or a series of flights and which finishes when the aircraft finally comes to rest and the engines are shut down at the end of the last flight on which he/she is a crew member.