The correct answer is:

B. Horizontal line

Explanation:

• A slope of zero indicates that the line does not rise or fall as it moves along the plane. Therefore, it must be horizontal.
• Horizontal lines are represented by equations of the form $y = c$, where $c$ is a constant.

Would you like further explanation or examples?

Relative questions to expand:

1. What is the equation of a line with an undefined slope?
2. How do you differentiate between vertical and horizontal lines geometrically?
3. Can a horizontal line pass through the origin? Why or why not?
4. How does slope relate to the angle of a line in a plane?
5. How would a slope of 1 or -1 appear on a graph?

Tip: Remember that slope is calculated as the "rise over run," so a slope of zero means no vertical change at all!