A disc magnet is used to hold a piece of paper (0.1 mm thick) to the steel door of a refrigerator. The magnet is 6 cm in diameter and 0.75 cm thick. Its mass is 500 grams. Assuming that the coefficient of friction between the paper and the refrigerator is 0.3, what is the minimum remanent magnetism that the magnet must have in order to keep the magnet and paper from falling

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

The object's initial distance above the groundthe object's initial velocity

The boat's velocity relative to the riverbank can be resolved into two components. the component going north is 2.7 m/s and the component going west is 1.2 m/s.we add these two vectors and then find the magnitude of the resultant to get the speed.the magnitude of the resultant vector from adding can be obtained by the pythagorean theorem.    to two significant figures, the boat's speed relative to the riverbank is 3.0 m/s.