Looking at the answer options:

Unavailability of CPDLC in the ATC sector --> INCORRECT. CPDLC (Controller - Pilot Data Link Communications) is a system that allows ATC and pilots to communicate by using text rather than by voice comms. If this system was unavailable, ATC would have to make voice calls instead, but not necessarily to change a call sign.

Similarity between two or more call signs --> CORRECT. If 2 aircraft are on the same frequency with similar call signs, then to prevent confusion, the Air Traffic Controller may instruct one of the aircraft to temporarily use a different call sign.

Duplicate transponder codes in the ATC sector --> INCORRECT. The problem is similar call signs, not transponder codes

Technical limitations of the ATC systems --> INCORRECT. This is due to similar call signs on the same frequency, not because of any technical limitation.

CAP 413
Callsigns for Aircraft

2.28 An aircraft shall not change its callsign type during a flight. However, where there is a likelihood that confusion may occur because of similar callsigns, an aircraft may be instructed by an air traffic service unit (ATSU) to change the type of its callsign temporarily.

2.29 The similarity of some aircraft callsigns on the same frequency can cause confusion which may lead to an incident. The following are particularly liable to be confused:

1. Callsigns which have in common three or more digits, especially when the flight numbers are the same, e.g. AIC 515 and SAS 515;
2. Aircraft with similar registrations, e.g. G-ASSB and G-ATSB or HB-SSB.

ICAO Annex 10
5.2.1.7.3 Radiotelephony procedures

5.2.1.7.3.1 An aircraft shall not change the type of its radiotelephony call sign during flight, except temporarily on the instruction of an air traffic control unit in the interests of safety.