Final answer:
The equation of a horizontal line passing through point M(a, b) is given by the equation y = b, representing a line where the y-value remains constant and parallel to the x-axis.
Step-by-step explanation:
The equation of a horizontal line that passes through a point M(a, b) is very straightforward. Since horizontal lines have a slope (m) of 0, there is no rise over run. The value of 'y' remains constant regardless of the value of 'x'. Thus, for every point on this line, the y-coordinate will be 'b', the same as the y-coordinate of point M.
In clear terms, the correct equation is: y = b. This indicates a line where every point's y-value is equal to 'b', and this line will be parallel to the x-axis. So, the answer to the student's question is b) y = b.