1. The problem asks: Which of the following is a solid figure? 2. A solid figure is a three-dimensional shape that has length, width, and height. 3. The options are: a. Square - a 2D shape with four equal sides. b. Triangle - a 2D shape with three sides. c. Cylinder - a 3D solid figure with two circular bases and a curved surface. d. Rhombus - a 2D shape with four equal sides but no thickness. 4. Since only the cylinder is a 3D solid figure, the answer is c. Cylinder. Final answer: c. Cylinder