What is the key phrase to inform pilots when two-way communications have been established in Class D airspace?

In Class D airspace, the phrase used to indicate that two-way communications have been established with the pilot is "(call sign) standby." This phrase serves multiple purposes:

1. Acknowledgment of Communication: It confirms to the pilot that their transmission has been received, and the controller is aware of their presence and intentions.

2. Indication of Workload: By saying "standby," the controller signals that they are occupied with other tasks or that additional time may be needed to process the request. This is crucial in busy airspace where effective communication is essential for safety and efficiency.

3. Before Further Instructions: It indicates to the pilot that additional instructions or clearances will follow but that they should hold on for the moment. This helps maintain order in communications, ensuring that pilots do not act prematurely before receiving necessary directions.

The other options, while they may be useful in different contexts, do not convey the same meaning regarding the establishment of two-way communications in Class D airspace. "Hold for clearance" indicates that the pilot needs to wait for a specific clearance before taking action, while "proceed with caution" and "cleared for takeoff" are specific instructions that would typically follow the initial communication establishment.