(d) is same at all points.
The field lines inside a solenoid are parallel straight lines, indicating a uniform magnetic field at all points inside it.
Source: Chapter 12, Section 12.2.4 – Magnetic Field due to a Current in a Solenoid
The passage explicitly states: "The field lines inside the solenoid are in the form of parallel straight lines. This indicates that the magnetic field is the same at all points inside the solenoid." Examiners expect you to select option (d) and may ask for a one-line justification — always mention "uniform" or "same at all points."