ANSWER
3) Radius of the wraps
Step-by-step explanation
We want to find the factor that will not affect the strength of a solenoid.
A solenoid is an electromagnet and there are four factors that mainly affect the strength of an electromagnet:
1) The wire size;
2) The number of loops/turns;
3) The current;
4) The presence of an iron core.
Therefore, the correct option is the radius of wraps/turns (option 3) since it does not affect the strength of a solenoid.