Answer: A cylinder of radius a with the z-axis as its axis.
- A A circle of radius a lying in the XY-plane only
- B A pair of planes parallel to the XY-plane
- C A cylinder of radius a with the z-axis as its axis
- D A sphere of radius a centred at the origin
Correct answer: C. A cylinder of radius a with the z-axis as its axis
Explanation: The condition x² + y² = a² places no restriction on z, so the point can sit at any height - sweeping out an infinite circular cylinder.
Concept context
Coordinate axes and planes in space, octants, distance between two points, and the section formula
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