Which of the following shapes has three sides?

The correct answer is a triangle because it is defined as a polygon that has exactly three sides. In geometry, a triangle can also be classified into various types based on the lengths of its sides or the measures of its angles, but the key characteristic that distinguishes it is the number of sides it possesses.

Other shapes listed, such as a square, rectangle, and pentagon, have four, four, and five sides respectively, which clearly differentiates them from a triangle. The emphasis on the number of sides is crucial when identifying basic geometric shapes. Understanding these fundamental properties aids in recognizing and classifying shapes effectively.