Why is mercury not suitable as the liquid in the U-tube? (the relationship between density of the liquid and the pressure in the liquid)​

Mercury Is more suitable than water cause it is denser than water.

Relation between density and pressure

Pressure = force/area

Replace force by mass * acceleration
Pressure = mass * acceleration/ area

We can replace mass by density * volume since density = mass/ volume
Pressure = density * volume * acceleration/ area

Volume is length^3 whereas area is length ^2. So volume / area = length

Pressure = density * length * acceleration

Gravity is form of acceleration so

Pressure = density * length * gravity
Length can be height or depth

Finally pressure = density * height * gravity

P = ρ g h => pressure of liquid

26.4 is about the right time

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