(d) an infinite number of solutions
When two lines are coincident, every point on the line is a common solution, giving infinitely many solutions.
Source: Chapter 3, Section 3.2
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The textbook clearly states: "If the lines coincide, then there are infinitely many solutions — each point on the line being a solution." This is a dependent and consistent pair. Examiners expect the correct option and may ask for a one-line reason, so always remember the three cases: intersecting → unique solution; parallel → no solution; coincident → infinite solutions.