Right answer is (d) Pulse repetition frequency
The explanation: LORAN stations are composed of a master station and two slave stations. Each pair typically broadcasted over the following frequencies of 1.75, 1.8, 1.85, or 1.9 MHz. However, the location of stations is such that it allowed a particular station to pick up signals from at least three stations. Thus to detect to pairs of signals, pulse repetition frequency comparison is employed.