Option A is correct. Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
The textbook explicitly states: "No two field-lines are found to cross each other. If they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible." This makes both A and R true, with R directly explaining A. Source: Chapter 12, Section 12.1 – Magnetic Field and Field Lines.