Which would be a stronger electromagnet, the one with the larger coils (a) or the smaller coils (b) ?

I believe the one with the smaller coils would be a stronger electromagnet. An electromagnet is a temporary magnet made from current carrying wire around a magnetic metal core. The strength of an electromagnet, is directly proportional to not only the current, but also the number of windings around the solenoid. Increasing the number of windings and making them smaller will increase the strength of an electromagnet.