Answer
a. No effect.
b. Magnetic field strength halved.
c. No net effect.
Work Step by Step
The magnetic field strength inside a solenoid is given by equation 20-8, $B=\frac{\mu_{o}NI}{L}$.
a. No effect. The magnetic field strength doesn’t depend on the solenoid diameter.
b. Magnetic field strength is halved. Doubling the spacing between the loops means doubling the length L of the solenoid.
c. No net effect. Doubling both N and L has no effect.
