Which of the following is mandatory to comply?

Airworthiness Directives (ADs) are legally enforceable regulations issued by aviation authorities to ensure the continued safety and airworthiness of aircraft. These directives require aircraft owners and operators to comply with specified measures that address safety issues, including mandatory inspections, modifications, or repairs. The nature of an AD implies that there is a recognized need for immediate action to rectify a condition that could adversely affect the safety of the aviation system. As such, compliance with ADs is not optional but a regulatory requirement, making them mandatory for all affected aircraft.

On the other hand, the other options, while important for maintenance and safety communications, do not carry the same legal weight. Service Bulletins (SBs) are recommendations from manufacturers to inform operators of improvements, modifications, or maintenance procedures but are not required unless they are codified into an AD. Similarly, Service Letters and Vendor Service Bulletins provide useful information and recommendations but lack the mandatory compliance nature of ADs. Understanding the distinctions between these documents is crucial for ensuring compliance with aviation regulations and maintaining aircraft safety.