(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
A is true: Two field lines cannot intersect because at any point there can be only one direction of the magnetic field; if lines crossed, two directions would exist at that point — a contradiction.
R is true: The textbook states the magnetic field at a given point depends directly on the current through the wire — more current, stronger field.
R does not explain A: The reason field lines don't intersect has nothing to do with how field strength varies with current; it is about the uniqueness of field direction at every point. Hence option (b) is correct.