Looking at the answer options:

Let the aircraft follow its clearance and ignore the lack of communication → INCORRECT. ATC will not ignore the lack of comms. Instead the controller will make other attempts at contact.

Use another aircraft to relay the message → CORRECT. If an aircraft on frequency can hear the target aircraft, then ATC may ask that aircraft to relay messages.

Try to make contact on a secondary frequency → INCORRECT. There is no ‘secondary frequency' as such. 121.5 is normally referred to as the emergency frequency. Having 121.5 on box 2 is a standard procedure, and it appears to be word play from the examiner.

Do not try to make contact since the pilots might be dealing with some critical emergency → INCORRECT. The controller has to proceed with lost comms procedures. If indeed the aircraft is dealing with an emergency, then the pilot would just ignore the radio.

ICAO Annex 10 Volume II
CHAPTER 5. AERONAUTICAL MOBILE SERVICE — VOICE COMMUNICATIONS

5.2.2.7.2 GROUND-TO-AIR

5.2.2.7.2.1 When an aeronautical station has been unable to establish contact with an aircraft station after calls on the frequencies on which the aircraft is believed to be listening, it shall:

a) request other aeronautical stations to render assistance by calling the aircraft and relaying traffic, if necessary;

b) request aircraft on the route to attempt to establish communication with the aircraft and relay traffic, if necessary.