Final answer:
To show that quadrilateral ABCD is a parallelogram, we can use the slopes of its sides. If opposite sides have the same slope, then they are parallel. The correct option is a.
Step-by-step explanation:
To show that quadrilateral ABCD is a parallelogram, we need to prove that opposite sides are parallel.
Given that the slope of AB is 4 and the slope of DC is 4, we can conclude that AB and DC are parallel because they have the same slope. Similarly, the slope of AD is -1 and the slope of BC is -1, which means AD and BC are also parallel.
Therefore, ABCD is a parallelogram. Hence the answer is a.