Explanation: To determine a "fix" position, which typically refers to a two-dimensional location, a receiver needs at least two intersecting lines of position (LOPs). In a LORAN (LOng RAnge Navigation) system, which operates on the principle of hyperbolic navigation, each LOP is generated by measuring the time difference of arrival (TD) between signals from a "master" station and a "slave" station. This master-slave combination forms one pair of transmitters that defines a hyperbola upon which the receiver is located. To obtain two distinct LOPs, which are necessary for a unique position fix, you need two such time difference measurements. These two measurements would ideally come from two independent pairs of "master and slave" stations. If each "pair" consists of a unique master and a unique slave, then two such pairs would involve a total of four distinct land-based transmitters (e.g., Master 1 and Slave 1 for the first LOP, and Master 2 and Slave 2 for the second LOP). While LORAN-C chains often used one master and multiple slaves to generate several LOPs (e.g., Master-Slave1 and Master-Slave2 using 3 transmitters total), the phrasing "two pairs of 'master and 'slave' stations" in option D implies two distinct and separate master-slave units, totaling four transmitters. This fulfills the requirement of two independent LOPs.