The correct answer is: "the reasonable belief that a suspect has committed a crime".
The probable cause was implemented by the Fourth Amendment to the US Constitution in order to limit the power of authorities and seeking to avoid unlawful search and seizures. The probable cause is the minimum standard required to consider that a person is a suspect, so that it becomes legal to perform a search in his/her domicile, for example. The authorities need to collect evidence if they want that a jury to accep that the probable cause is fulfilled, before they can act against a subject.