The horizontal line in the graph is called _______.
a. x – axis
b. y – axis
c. Line
d. None of these

Hint: Draw a horizontal line in a Cartesian plane. Compare it to the x – axis and y – axis and find out in the graph what the horizontal line represents.

Complete step-by-step answer:

A horizontal line is one that goes from left to right. They are parallel to the x –axis in the coordinate plane. A horizontal line has a slope of zero. As you move along the right of the line, it does not rise or fall at all.
If we consider an example of point (5, 2) and (15, 2) then we can mark the point in the Cartesian plane.

The equation of a horizontal line is y = b, where x is the coordinate of any point on the line, where the line b crosses the y – axis.
The two points A, B on the line are at (5, 2) and (15, 2). The second coordinate in each pair is the y – coordinate which are 2 and 2. Since they are equal the line is horizontal.
Thus the horizontal lines in the graph are called the x –axis.
\[\therefore \] Option (a) is the correct answer.

Note: In the interest of clarity, the coordinates are rounded off to integers and the lengths are rounded to one decimal place. This can cause calculations to be slightly off.