Choice "D" is correct. A data librarian is the most appropriate position to maintain custody of an entity's data. As the name implies the librarian maintains custody of the "library" of data generated by an organization.
Choice "b" is incorrect. A systems analyst is the position that designs the overall application system. The systems analyst has, effectively, an authorization role that should be segregated from the custody role.
Choice "a" is incorrect. A computer operator is responsible for scheduling processing jobs, etc., and would not have actual custody of the data. Effectively the operator is in a recordkeeping function that should be segregated from custody.
Choice "c" is incorrect. A computer programmer may be either an application programmer responsible for writing or maintaining application programs or a system programmer responsible for installing supporting monitoring and maintaining the operating system. Programmers have a recordkeeping function that should be segregated from the librarian's custody function.