In order to solve a linear system graphically, we need to determine the points at which the lines that represent the system's equations intersect. The solution to the system are the intersection points, so we have three possibilities:
1 - The lines never touch, they are parallel. For this situation, we don't have a solution.
2- The lines touch once. The point of intersection is the solution.
3- The lines are coincident, which means that they are on top of each other. Any given point on the lines will be considered a solution, therefore there is an infinite number of solutions.
For our case the lines are on top of each other, so the correct answer is 1 - Infinite set of points on the line.