The correct answer is D. FSK.
A linear modulation technique is a modulation technique in which the modulated signal is a linear function of the input signal. In other words, the output signal is a linear combination of the input signal and a set of basis functions.
BPSK, QPSK, and OQPSK are all linear modulation techniques. In BPSK, the input signal is a binary signal (either 0 or 1), and the output signal is a sine wave with a frequency that is modulated by the input signal. In QPSK, the input signal is a two-bit signal (either 00, 01, 10, or 11), and the output signal is a sine wave with a frequency that is modulated by the input signal. In OQPSK, the input signal is a four-bit signal (either 000, 001, 010, 011, 100, 101, 110, or 111), and the output signal is a sine wave with a frequency that is modulated by the input signal.
FSK is a non-linear modulation technique. In FSK, the input signal is a binary signal (either 0 or 1), and the output signal is a sine wave with a frequency that is either one of two frequencies, depending on the input signal.
The reason why FSK is not a linear modulation technique is because the output signal is not a linear combination of the input signal and a set of basis functions. The output signal is a sine wave with a frequency that is either one of two frequencies, depending on the input signal. This is not a linear combination of the input signal and a set of basis functions.