Final answer:
ELT batteries should be replaced according to the intervals recommended by the ELT manufacturer to ensure their reliability in emergencies.
Step-by-step explanation:
The question relates to the maintenance procedures for emergency locator transmitters (ELTs), which are critical safety devices used in aviation. According to the question, ELT batteries shall be replaced at intervals recommended by the ELT manufacturer. This is the correct answer, as manufacturers provide specific guidelines on replacement intervals to ensure the device remains functional in case of an emergency. It is important to follow the manufacturer's recommendations, rather than relying on the aircraft's manufacturing date, international regulations alone, or regional weather conditions.