In an inverting op-amp configuration, the input and output signals are:

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In an inverting op-amp configuration, the output is 180 degrees out of phase with the input. This happens because negative feedback pulls the inverting input to a virtual ground (near zero volts). When the input voltage rises, the op-amp drives the output in the opposite direction (negative) to keep the inverting input at ground. Conversely, a falling input causes the output to rise. So the signal at the output swings in the opposite direction to the input, giving a full 180-degree phase difference. If it were non-inverting, the output would remain in phase with the input. The other phase options don’t match the behavior of an ideal inverting amplifier with negative feedback.