All horizontal lines have a slope a.)zero b.)one half c.)one d.)undefined

Quick answer A <<<<

All horizontal lines have an equation of 
y = k where k is a number. k is the y intercept.
Here are a few lines drawn for you.

y = 3
y = 1
y = - 1
y = - 2

In y = ax + b the term ax disappeared. The graph shows that x is not zero. It  has many values. So the m must be zero.

A <<<<answer