The key to identifying a black hole candidate in a binary system is that a. one of the two stars cannot be seen. b. the unseen star is a contact binary. c. the visible companion must be an evolving main sequence or giant star. d. the system must be a very strong source of radio emissions.

D. Black holes due to their nature are extremely hard to directly observe, they do however emit radio emissions. We can use clues from the surrounding space to detect them. A is not correct as black holes do not necessarily block other stars light. B is false because contact binaries are not exclusive formations from black holes. C is false because not all stars create black holes, giant stars may but finding the star doesn't mean you've found a black hole yet.