What is the symbol for a NOT gate?

In digital logic design, a NOT gate, also known as an inverter, is represented by a triangle with a small circle at the output. The triangle indicates the direction of the signal flow, while the small circle signifies negation or inversion of the input signal. When the input to a NOT gate is true (or high, represented by a 1), the output will be false (or low, represented by a 0), and vice versa. This graphical representation effectively conveys the function of the NOT gate, making it clear that it produces an output that is the opposite of its input.

The other representations provided do not align with standard symbols used in logic circuit design for a NOT gate. They might depict other types of gates, but none represent the negation functionality inherent to the NOT gate.