C

Step 1: Understand the magnetic field inside a long solenoid. The magnetic field () inside an ideal (long and tightly wound) solenoid is given by the formula:
$$ B = imes n imes I $$
where

• $B$ = Magnetic field inside the solenoid,
• $ $ = Permeability of free space,
• $n$ = Number of turns per unit length,
• $I$ = Current flowing through the solenoid.

Step 2: Analyze the parameters affecting the magnetic field.
The formula clearly indicates that the magnetic field inside the solenoid depends on:
- The number of turns per unit length ($n$)
- The electric current flowing ($I$)
Step 3: Consider the role of the radius of the solenoid.
The magnetic field strength inside a long solenoid is independent of its radius as long as we are considering an ideal solenoid. Thus, for an ideal solenoid, changing the radius does not affect the magnetic field strength, as the field is uniform and concentrated within the solenoid.
Conclusion: Therefore, the radius of the solenoid does not affect the magnetic field produced inside, while the other two factors do. Hence, the correct answer is C.