During transcranial magnetic stimulation (TMS) treatment, a magnetic field typically of magnitude 0.55 T is produced in the brain using external coils. During the treatment, the current in the coils (and hence the magnetic field in the brain) rises from zero to its peak in about 85 μs . Assume that the magnetic field is uniform over a circular area of diameter 2.1 cm inside the brain. What is the magnitude of the average induced emf | E | around this area in the brain during the treatment?

Ohm's law tells you that i = v/r where v is equivalent voltage and r is equivalent resistance of the circuit
since all circuits have the same voltage, your question is really asking which circuit has the lowest total resistance, since the lower the resistance, the higher the current
circuit a has a resistance of 5 ohms; circuit c is in series so has a resistance of 10 ohms; circuit b is in parallel and therefore has a resistance of 2.5 ohms (review your notes on adding resistances in parallel and series); therefore b has the highest current

Answershe should not have multiplied the sulfur atoms by the subscript 4