Answer
A parallelogram is formed.
Work Step by Step
We assign points to each of the midpoints:
$A: a,0 \\ B: b,c \\ C: a+2b, 2c \\ D: 2a+b,c$
We now find the slopes of the lines between the midpoints:
$m_1 = \frac{c}{b-a}$
$m_2 = \frac{c}{b+a} $
$m_3 = \frac{c}{b-a} $
$m_4= \frac{c}{b+a} $
Since opposite sides are parallel, we see that it is a parallelogram.
