In the common emitter amplifier, when the input and output signals are compared:

In a common emitter amplifier, the input signal is applied to the base terminal and the amplified output is taken from the collector. When the input voltage increases, the forward bias on the base-emitter junction increases, causing a proportionately larger collector current to flow. As this collector current increases, the voltage drop across the collector load resistor also increases. Because the total supply voltage is fixed, a larger voltage drop across the load resistor leaves less voltage at the collector terminal itself. Consequently, a positive-going input produces a negative-going output, resulting in a 180-degree phase inversion. Other options are incorrect because in-phase signals are characteristic of common base and common collector amplifiers. Furthermore, 90-degree phase shifts are caused by reactive components (inductors and capacitors), not by the inherent topology of a BJT amplifier.