The strength of magnetic field produced inside a long straight current carrying solenoid does not depend upon :

The NCERT chapter explains (Section 12.2.4 and 12.2.1–12.2.3) that the strength (magnitude) of the magnetic field produced inside a current-carrying solenoid depends on the number of turns, the current through it, and the material of the core placed inside it — but reversing the direction of current only reverses the direction of the field, it does not change its strength (magnitude).

• (A) DOES affect strength — the chapter states "if there is a circular coil having n turns, the field produced is n times as large as that produced by a single turn" since the field due to each turn adds up; a solenoid is essentially many such coaxial turns, so more turns give a stronger field.
• (B) does NOT affect strength (correct answer) — Activity 12.4 shows that reversing the current direction only reverses the direction of deflection of the compass needle (i.e., reverses the field direction); the magnitude of the field is governed by current magnitude, turns and core, not by which way the current flows.
• (C) DOES affect strength — the chapter states a strong magnetic field inside a solenoid "can be used to magnetise a piece of magnetic material, like soft iron, when placed inside the coil," forming an electromagnet; a magnetic core (like soft iron) concentrates and strengthens the field compared to an air core.
• (D) DOES affect strength — a larger radius spreads the same current-produced field over a larger area, weakening the field per unit area at a given point (analogous to how field strength around a straight wire decreases with distance, so radius/geometry of the coil affects the field magnitude at the axis).