(a) — Parallel straight horizontal lines with arrows pointing right (uniform field inside the solenoid).
The field lines inside a current-carrying solenoid are parallel straight lines, indicating a uniform magnetic field throughout its interior.
Source: Chapter 12, Section 12.2.4
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Examiners expect you to identify option (a) and state the key reason: field lines inside a solenoid are parallel and straight, showing the field is uniform. Option (b) is for a straight wire (concentric circles); option (c) may look like a solenoid shape but is incorrect; option (d) resembles the external field of a bar magnet. The textbook explicitly states: "The field lines inside the solenoid are in the form of parallel straight lines."