What is a counterexample? I kind of forgot so can anyone give sentence examples?

Fill in the blank 1. a rhombus is a rectangle a) always b) sometimes c) never 2. a square is a quadrilateral a) always b) sometimes c) never 3. a trapezoid is a kite a) always b) sometimes c) never 4. a quadrilateral is a kite a) always b) sometimes c) never 5. a square is a rhombus a) always b) sometimes c) never 6. a parallelogram is a rectangle a) always b) sometimes c) never