What does the following symbol represent?

Which of the following relations are correct according to Boolean algebra?

$A$ building has two overhead water tanks, each fitted with a sensor $(S_1, S_2)$ which goes to $0$ when the water level in the tank falls below a set value and remains $1$ otherwise. $A$ common pump is used to raise water from an underground storage tank to these overhead tanks. Of the following circuits, which one will turn on $(P = 1)$ the pump only when at least one of the tanks has a water level below the set value?

The truth table for the circuit given below is $:$

Which of the following gates will have an output of $1$?

What does the following combination of gates produce?