What was the official instrument used to govern the Cayman Islands before the 1959 constitution?

The official instrument used to govern the Cayman Islands before the 1959 constitution was indeed an act to regulate the Legislative Assembly of the Cayman Islands. This act served as the primary legal framework guiding the governance of the islands, outlining the structure and functions of the Legislative Assembly. Prior to the introduction of the 1959 constitution, the governance was not driven by a constitution that provided detailed civil liberties or a clear separation of powers, but rather by this regulatory act which set forth how the local government would operate within the context of British colonial rule.

The other options do not accurately reflect the governing structure prior to the constitution. The Jamaican Constitution, while relevant during the time when the Cayman Islands were closely associated with Jamaica (before gaining their own constitutional advancements), was not a governing document specific to the islands. The local law code refers to various statutes and legal frameworks but does not encapsulate the overarching governance instrument like the act does. The British Commonwealth Charter represents broader principles and values governing member states but was not specific to the localized administration of the Cayman Islands before the 1959 constitution.