To answer this question, you need to understand different testing techniques and their applicability.
Option A) Specification-based technique - This technique involves testing the system based on the specifications or requirements provided. It may not be suitable for testing changes on old and undocumented functionalities because there may not be clear specifications or requirements available for those functionalities.
Option B) Black box technique - This technique involves testing the system without having knowledge of its internal structure or implementation details. It may be suitable for testing changes on old and undocumented functionalities since it focuses on the system's external behavior and functionality.
Option C) White box technique - This technique involves testing the system with knowledge of its internal structure and implementation details. It may be the most appropriate technique for testing changes on old and undocumented functionalities because it allows the tester to understand the underlying code and make targeted tests.
Option D) Data-driven testing technique - This technique involves testing the system using different sets of input and output data. While it can be used to test changes on old and undocumented functionalities, it may not be the most appropriate technique since it does not specifically address the need to understand the system's internal structure.
Based on the explanations above, the correct answer is C) White box technique. This technique is most appropriate for testing changes on old and undocumented functionalities as it allows the tester to have knowledge of the system's internal structure and implementation details.