1. The problem asks: Which of the following is a solid figure? 2. The options are: a. Square b. Triangle c. Cylinder d. Rhombus 3. A solid figure is a three-dimensional shape that has length, width, and height. 4. Among the options, a square, triangle, and rhombus are flat, two-dimensional shapes (plane figures). 5. A cylinder is a three-dimensional solid figure with circular bases and height. 6. Therefore, the correct answer is **c. Cylinder**. Final answer: c. Cylinder