NOT logic is formed by which of the following?

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Inversion of the input signal is what gives a NOT operation in relay-type logic. A normally closed contact provides this behavior because it conducts when the input is not energized, but opens and stops current when the input is energized. That means the output is the opposite of the input: on when the input is off, and off when the input is on. The other configurations don’t produce a single-input inversion: a normally open contact conducts only when the input is on, mirroring the input rather than inverting it; two normally closed contacts in parallel depend on more than one input and don’t guarantee a true inversion for one signal; two normally open contacts in series require both inputs to be on, which is an AND condition, not an inverter. So a single normally closed switch best implements the NOT behavior.