Which form needs to be completed before placing air inbound cargo in the appropriate bay?

The correct choice is AMC Form 156. This form is specifically designed for documenting the delivery of cargo and ensuring that all necessary information is collected before placing air inbound cargo in the appropriate bay. By completing this form, personnel verify that the cargo meets all airlift requirements and is ready for processing, which helps facilitate efficient operations and minimize errors during the handling of incoming shipments.

The other forms mentioned serve different purposes within logistics and transportation but are not specifically tailored for the initial documentation needed for inbound air cargo placement. DD Form 2775 is used for documentation of a shipment's weight and balance, DD Form 1387 is related to the marking and labeling of military shipments, and SF 30 is typically utilized for contract modifications. Each serves important roles but does not directly fulfill the requirement of processing inbound cargo into the appropriate air transport facility.