Step 1: Meaning of a key informant.
A key informant is a person who possesses extensive and reliable knowledge about a community or social setting.
Step 2: Role in sociological research.
Such individuals help researchers understand social norms, practices, power relations, and cultural meanings from within the society.
Step 3: Evaluation of options.
Restricting access is not a defining role of a key informant, while deep community knowledge and social position are essential characteristics.
Step 4: Conclusion.
Hence, options referring to in-depth knowledge and socially informed understanding are correct.
Final Answer: (B), (D)