Explanation:

- Option: 1, Investigation: This refers to a systematic and methodical process to discover facts and gather evidence about a crime. It involves detectives or law enforcement officers actively working to solve a case.

- Option: 2, Detection: This involves identifying or discovering something that was not initially visible or known. In crime, it often refers to the act of uncovering illegal activities or perpetrators.

- Option: 3, Sixth-sense: This term describes an intuitive feeling or hunch that goes beyond the five traditional senses. It’s often portrayed as a mysterious ability to perceive things that are not obvious.

- Option: 4, Serendipity: This describes the occurrence of making fortunate discoveries by accident or luck. In the context of crime, it refers to solving a crime more by chance than by intention.

- Accidental solving of a crime often aligns with the concept of serendipity because it implies that the resolution came about unexpectedly rather than through planned action.

Correct Answer: Option: 4, Serendipity