How can call signs be abbreviated on a transmission after communications have been established?

The correct choice highlights that after communications have been established, it is appropriate to abbreviate call signs by using the last three digits or letters of the aircraft identification. This method provides clarity and maintains efficiency in communications.

Abbreviating to the last three characters allows air traffic controllers and pilots to recognize each other quickly without losing essential identification, especially in busy or complex environments. This practice enhances situational awareness while reducing the potential for misunderstanding or miscommunication.

The other methods, such as stating only the first letter or using squawk codes, might lead to confusion or provide insufficient information for identification. Simply saying the numbers without context may also create ambiguity, especially in radio transmissions where clarity is paramount. Thus, option A represents the most effective and standard practice in air traffic communications post-establishment.