Which of the following is a feasible region?

A) Square with corners at (0, 0), (1, 0), (1, 1), and (0, 1)
B) Circle with center at (0, 0) and radius 1
C) Triangle with vertices at (0, 0), (1, 0), and (0, 1)
D) Rectangle with corners at (0, 0), (2, 0), (2, 1), and (0, 1)