We can't directly observe black holes, but we can observe how they impact nearby objects. The presence of stellar-mass black holes (i.e., produced by the deaths of massive stars) is inferred from
A. radio emission, as they rotate really rapidly due to conservation of angular momentum
B. we have not yet detected or inferred the existence of stellar-mass black holes.
C. hot, X-ray emitting disks of gas being accreted from a companion by the black hole.
D. the orbits of stars and planets around the black hole.