Question: "What should someone ideally have to alter the DMBS"

Context: "Many other databases have application software that accesses the database on behalf of end-users, without exposing the DBMS interface directly. Application programmers may use a wire protocol directly, or more likely through an application programming interface. Database designers and database administrators interact with the DBMS through dedicated interfaces to build and maintain the applications' databases, and thus need some more knowledge and understanding about how DBMSs operate and the DBMSs' external interfaces and tuning parameters."

Answer:
knowledge and understanding about how DBMSs
The question is asking for the ideal thing that someone should have to alter (i.e., modify) a database management system, given the context of application programmers using an interface and database designers/administrators needing some more knowledge and understanding about how DBMSs operate etc..
Based on this information, it can be inferred that in order to successfully modify a DBMS, one would need "knowledge and understanding" of its interfaces as well its operational parameters like tuning..