Regulation (EU) No 376/2014
Article 4 Mandatory reporting

1. Occurrences which may represent a significant risk to aviation safety and which fall into the following categories shall be reported by the persons listed in paragraph 6 through the mandatory occurrence reporting systems pursuant to this Article:

(a) occurrences related to the operation of the aircraft, such as:

(i) collision-related occurrences;
(ii) take-off and landing-related occurrences;
(iii) fuel-related occurrences;
(iv) in-flight occurrences;
(v) communication-related occurrences;
(vi) occurrences related to injury, emergencies and other critical situations;
(vii) crew incapacitation and other crew-related occurrences;
(viii) meteorological conditions or security-related occurrences;

As for the options:
Crew emergency. ---> INCORRECT. A “crew emergency” (for instance, incapacitation or in-flight distress declared by the crew) clearly affects safety and therefore must be reported under the mandatory reporting system.

Passengers not paying attention to the safety instructions. ---> CORRECT. This does not meet the threshold of an occurrence that directly endangers flight safety but may be valuable for safety culture or human factors monitoring. This kind of situation is exactly what voluntary reporting aims to capture; behavior, complacency, or safety attitude issues not otherwise reportable.

CB Cloud. ---> INCORRECT. Encounters with severe weather phenomena (like CB clouds, turbulence, wind shear, etc.) that affect flight safety must be reported mandatorily.

Passenger injury. ---> INCORRECT. Any injury sustained by a passenger during flight (for example, due to turbulence or accident) is a mandatory occurrence under Regulation.

Note: A detailed list of all mandatory reporting occurrences can be found in EASA (EU) Regulation 2015/1018. This list includes critical occurrences for the safe conduct of the flight, and occurrences related to the passengers not paying attention to the safety demonstration are not included. Therefore, this is considered as 'Voluntary Reporting'.