A discrete signal is defined as:

• A. For all real time.
• B. It is always periodic.
• C. At discrete values of time.
• D. Only at t = 0.

Answer: (C)

In signals and systems, a discrete signal is defined only at specific time instants, usually at integer multiples of a sampling period. The values exist for those discrete times, and there’s no defined value for every real time.

That’s why the statement that the signal is defined at discrete values of time is the correct way to describe a discrete signal. If it were defined for all real time, it would be a continuous-time signal. Saying it’s always periodic isn’t a requirement—discrete signals can be nonperiodic. And being defined only at a single moment like t = 0 would be far too restrictive; a discrete signal consists of a whole sequence of samples, such as x[n] for n = 0, 1, 2, … (and possibly extending to negative indices for a bi-infinite sequence).