The correct answer is B) Four. A GPS receiver needs to receive signals from at least four satellites in order to determine an exact three-dimensional position. This is because the GPS receiver needs to solve for four unknown variables: the x, y, and z coordinates of the receiver's position, as well as the time offset of the receiver's clock compared to the satellite clocks. The other answer choices are incorrect because three satellites would not provide enough information to determine all four unknown variables, and five or more satellites would provide redundant information beyond what is necessary to calculate the position.