a
Undefined
b
Negative
c
Zero
d
Positive

A, undefined

Step-by-step explanation:

The general rule is that vertical lines are undefined slope, because there's literally no change in x values. The vertical line test also fails, meaning there is no slope. Horizontal lines have a slope of 0, while vertical lines don't own any numbers that are important except for x-axis numbers.

If you do y2-y1 over x2-x1, you literally get 1/0 which is nothing. 0/1 = 0, but 1/0 is nothing.

Hope this helps!

if you explain the the question more correctly i might be able to answer this better

step-by-step explanation:

fist on the left goes with the third on the right. second on the left goes with the fourth on the right. third on the left goes with the second on the right. the fourth on the left goes with first on the right