Answer:
x-axis
Explanation:
a line in the cartesian plane is defined by the general equation:

where
is the slope and
is the y-intercept, so for a given
value this return a value for
.
When the slope of a line is equal to zero (
) we have:
(
has always the same constant value)
which means that the value of
for any given
is the same, we have a constant function, that is represented in the plane as a horizontal line.
So when the slope is zero the line is a horizontal line, which is parallel to the x-axis since the x-axis is also a horizontal line.