Which of the Following Constitutes a Stop

Which of the following constitutes a stop?

A) A person approaches an officer and says,"I saw a man run out of that building with a knife dripping blood."
B) Police approach a person and ask,"Did you just leave that building?"
C) A person walks up to an officer and volunteers,"I just killed my enemy."
D) Officers investigating a robbery that just happened approach a person who fits the description given by the victim,asking who they are where they were at the time of the crime.