Which area is identified by the letter "T" and is available only during the day?

The area identified by the letter "T" that is available only during the day corresponds to VFR (Visual Flight Rules) Transit lanes. VFR Transit lanes are designated routes through controlled airspace that provide pilots flying under VFR with a way to navigate while minimizing the risk of conflict with IFR (Instrument Flight Rules) traffic.

These lanes are specifically established for daytime use, where visual reference is possible. This aspect is crucial as it aligns with the need for pilots to maintain visual separation from other aircraft and to have a clear view of the terrain and airspace around them. During nighttime, such transit lanes would typically not be used for safety reasons, as the visual references required for VFR flight are limited.

In contrast, Control Zones are areas of controlled airspace typically surrounding an airport that are active at all times, not limited to daytime. General Aviation Areas might permit operations at all hours as well, depending on local regulations, and Parachute Landing Areas can be used at different times based on the specific operations taking place there. Therefore, the defining characteristics of VFR Transit lanes highlight their availability only during daytime operations.