The correct answer is C) 3 theoretical position lines are required for a two-dimensional fix using GPS. To determine a two-dimensional position using GPS, the receiver must calculate its distance from at least three satellites. This allows the receiver to intersect three spherical position lines, resulting in a two-dimensional fix on the Earth's surface. The other options are incorrect because a single position line (A) would only provide direction but not distance, two position lines (B) would result in two possible locations, and four position lines (D) would be redundant for a two-dimensional fix.