Explanation:

This question requires specific knowledge of the definitions provided in the Juvenile Justice (Care and Protection of Children) Act, 2015 (JJ Act, 2015). The 'best interest of the child' is a foundational principle of this Act and child rights jurisprudence globally, making its precise definition crucial. **Analysis of Options:** * **Option (1) Section 2(5):** Section 2 of the JJ Act, 2015, is dedicated to definitions. Section 2(5) defines 'authorised person'. This is not the definition for 'best interest of the child'. Therefore, this option is incorrect. * **Option (2) Section 2(9):** This is the correct answer. Section 2(9) of the Juvenile Justice (Care and Protection of Children) Act, 2015, defines "best interest of the child" as "the basis for any decision taken regarding the child, to ensure fulfilment of his basic rights and needs, identity, social well-being and family welfare, and protection from abuse, neglect and exploitation, and includes giving due consideration to the child's opinion." This comprehensive definition guides all authorities and institutions working under the Act to prioritize the holistic well-being and rights of the child in every decision-making process. It is a cornerstone principle that informs the interpretation and application of the entire Act. * **Option (3) Section 2(14):** Section 2(14) of the JJ Act, 2015, defines 'child in conflict with law'. This is a critical definition for the Act but is not the definition for 'best interest of the child'. Therefore, this option is incorrect. * **Option (4) Section 2(12):** Section 2(12) of the JJ Act, 2015, defines 'child friendly'. This term is related to the environment and procedures suitable for children but is not the definition for 'best interest of the child'. Therefore, this option is incorrect. In summary, the principle of 'best interest of the child' is explicitly and comprehensively defined in Section 2(9) of the Juvenile Justice (Care and Protection of Children) Act, 2015, serving as a guiding light for all actions taken under the Act.