Explanation: GPS satellites transmit on specific L-band frequencies. The L1 carrier operates at 1575.42 MHz and carries both the Coarse/Acquisition (C/A) code, which is the Standard Positioning Service (SPS) signal available to all users, and the crucial navigation message containing satellite ephemeris, almanac, and timing data. The L2 carrier operates at a different frequency, 1227.60 MHz, and primarily carries the P(Y) code, used for more precise positioning and by military receivers. Option D accurately describes the L1 carrier frequency of 1575.42 MHz and correctly states that it carries the navigation message and SPS code signals. This is the fundamental signal used by civilian GPS receivers. Options A and C present incorrect frequencies or misattribute the functions of the carriers. Option B is incorrect because GPS signals are transmitted on two *separate* carriers, not combined into a single intermediate signal for transmission.