Slope represented as a ratio of rise over run. In this case, we have a straight horizontal line. No matter how far we go along the line, there will be no rise, so the slope is a constant ratio of
[tex] \frac{0}{1} [/tex]
since for every one unit you move left or right, you are moving zero units up or down.
Any fraction with a numerator of 0 equals 0, so we can simplify this to say that the slope of our line is 0.
Choice 3 is correct.
Hope that helped! =)