The correct option is B 24
The global positioning system (GPS) is a satellite-based navigation and surveying system for the determination of precise position and time, using radio signals from the satellites, in real-time or in post-processing mode.
The GPS consists of 24 satellites in nearly circular orbits at about 20,200 km altitude.
GPS provides full coverage with signals from a minimum of 4 satellites available to the user, at any place on the earth.