Which of the following modifications to a solenoid would be most likely to decrease the strength of its magnetic field?

By reading the fine details of the question, carefully and analytically, I have determined that there's no list of modifications to choose from.

The strength of the magnetic field of a solenoid depends on the electric current in its coil windings, the number of wire turns in its coil windings, and the material in its core.

In order to DEcrease the strength of its magnetic field, any one or more of these steps could do the job:

-- DEcrease the electric current in its coil windings.  This can be accomplished by decreasing the voltage of the power source that energizes the coil, and/or increasing the resistance of the wire in the coil.

-- DEcrease the number of wire turns in the coil.

-- If the solenoid has anything in its core, change the core to something with a lower magnetic 'permeability'.  An Iron core will produce the greatest magnetic field strength.  Air, vacuum, or NO core will produce the lowest magnetic field strength.

wavelength divides in half and speed stays the same.