If you are “retrofitting” that means you are adding to an existing database management system (DBMS). You could go back and redesign the entire DBMS but the cost of that could be expensive and there is no telling what the effect will be on existing applications, but that is redesigning and the question states retrofitting. The most cost effective way with the least effect on existing applications while adding a layer of security on top is through a trusted front-end.
Clark-Wilson is a synonym of that model as well. It was used to add more granular control or control to database that did not provide appropriate controls or no controls at all. It is one of the most popular model today. Any dynamic website with a back-end database is an example of this today.
Such a model would also introduce separation of duties by allowing the subject only specific rights on the objects they need to access.
The following answers are incorrect:
trusted back-end. Is incorrect because a trusted back-end would be the database management system (DBMS). Since the question stated “retrofitting” that eliminates this answer.
controller. Is incorrect because this is a distractor and has nothing to do with “retrofitting”.
kernel. Is incorrect because this is a distractor and has nothing to do with “retrofitting”. A security kernel would provide protection to devices and processes but would be inefficient in protecting rows or columns in a table.