What may be issued before the execution of the basic owner-contractor agreement?

The correct choice is addenda. Addenda are modifications or clarifications to the original bid documents and are typically issued to provide additional information or to alter the terms of the contract before the execution of the owner-contractor agreement. They are crucial during the bidding process as they can help ensure that all parties are working with the same set of instructions and expectations.

For example, if there are changes in the design, scope, or schedule after the initial bid documents have been released, addenda allow these changes to be formally communicated to all potential bidders. This ensures all contractors are aware of and can price their bids accordingly.

On the other hand, change orders are issued after a contract has been executed and are used to amend the contract's provisions, whereas bid specifications encompass the requirements and guidelines for submitting bids but are typically not issued once bidding has closed. Subcontractor agreements refer to contracts established between the general contractor and subcontractors for specific parts of the work and would also be established after the owner-contractor agreement is in place.