The power of a private person to arrest is given under Section 43 of the CrPC. According to Section 43(1), any private person may arrest or cause to be arrested any person who in their presence commits a non-bailable and cognizable offence, or any proclaimed offender. The requirement is twofold for an ongoing offence: it must be both non-bailable (serious) and cognizable (allowing police to arrest without a warrant). Option (b) is the most accurate description of this requirement. Option (d) is too broad as it says "commits offence" which could include minor, non-cognizable offences for which this power is not available.