A charge Q is placed at each of two opposite corners of a square of length a. A charge q is placed each of the other two corners. Of the resultant electric force on the two charges Q is zero, what is the mathematical relationship between Q and q

Wooden , walls ,doors ,support

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

Since the pully is a simple machine so the mechanical advantage is a 1