Answer
It is a parallelogram.
Work Step by Step
We must prove that all lines are parallel to the lines opposite to them. From observation, we can see:
$m_1= 0$
We find the other slopes using the slope formula:
$m_2 = \frac{c-c}{a+b-a} =0$
$m_3 = \frac{c-0}{b-0} =c/b$
$m_4 = \frac{c-0}{a+b-a} =c/b$
Since slopes one and two are equal and slopes three and four are equal, it is a parallelogram.