Sections of the Code of Criminal Procedure: Section 4(1)(f) of the Code of Criminal Procedure, 1898 defines 'cognizable offence'.

​What is an actionable offence: It is a type of offense where the police can arrest the accused and start investigation without the prior permission (warrant) of a magistrate or court.
Application to Kidnapping: Since Kidnapping is a 'cognizable' or cognizable offense under the law, the police are legally bound to register a case or FIR and start investigation on receiving a complaint.
This is why instead of just doing a general diary (GD),In cases of abductions, there is an emphasis on lodging an FIR so that the police are bound to initiate an investigation