The correct answer is A. Single ended mode.
A differential amplifier is an electronic amplifier that amplifies the difference between two input signals. When either one of the inputs to the differential amplifier is equal to zero, then the amplifier is said to be operated in single ended mode. In this mode, the amplifier amplifies the difference between the input signal and a reference voltage, which is usually ground.
The other options are incorrect because they do not describe the mode of operation of a differential amplifier when either one of the inputs is equal to zero. Option B, differential mode, is the mode of operation when both inputs to the differential amplifier are different from zero. Option C, non inverting mode, is the mode of operation when the input signal is applied to the non inverting input of the differential amplifier. Option D, inverting mode, is the mode of operation when the input signal is applied to the inverting input of the differential amplifier.