What prefix is used when a controller establishes initial communications with U.S. registered civil aircraft?

The prefix used when a controller establishes initial communications with U.S. registered civil aircraft is "N," representing "November." This prefix is part of the phonetic alphabet used in aviation and is designated to identify American civil aircraft. When a pilot registers an aircraft in the United States, the registration number begins with the letter N followed by a series of numbers and possibly letters.

Using "N" allows for clear identification and communication over radio frequencies, which is crucial for safety and efficiency in air traffic control. The uniqueness of this prefix also helps to distinguish U.S. aircraft from those registered in other countries, each of which has its own prefix or designation. Other options provided do not represent prefixes used for U.S. civil aircraft registration, as they correspond more to state abbreviations or do not align with the regulatory framework used for aircraft identification.