An electromagnet is created using a battery , an insulated copper wire and and an iron nail . the wire is wrapped around the nail 20 times and attached to the battery. which pair of changes will definitely increase the strength of the electromagnet

C)Increase the voltage of the battery and increase the number of turns of the coil. 

According to the ideal transformers equation Ns/ Np = Vs/Vp where s= secondary and p= primary, number of turns is proportional to voltage so a increased number of turns and increased voltage result in a higher power.

Hope I helped!