Explanation: Marker beacons, an integral part of an Instrument Landing System (ILS), all operate on a common carrier frequency of 75 MHz. This standardization allows aircraft to use a single receiver to detect all three types of beacons (outer, middle, and inner). The distinction between the outer, middle, and inner markers is achieved not by unique carrier frequencies, but by different audio modulation tones and distinct Morse code identification patterns heard by the pilot. For example, the outer marker uses a 400 Hz tone, the middle a 1300 Hz tone, and the inner a 3000 Hz tone. Option A is incorrect because the 75 MHz frequency is universal, not unique to each airport. Option B incorrectly states a 400 MHz carrier, which is a UHF frequency, while marker beacons operate at 75 MHz (VHF). Option D is incorrect as 3000 kHz (3 MHz) is significantly lower than the actual 75 MHz operating frequency.