Analog and digital both signals are used to transmit information, usually through electric signals. In both these technologies, the information such as any audio or video is converted into electrical signals.
In analog technology, information is translated into electric pulses of varying amplitude from zero to maximum. In digital technology, translation of information is into binary formal (zero (0) or one (1)) where each bit is representative of two distinct amplitudes, these amplitudes varying either zero $(0)$ or $\max$ value (1).
So, (a) \& (b) would produce analog signal and (c) \& (d) will produce digital signals.