A “flat” is a set of rooms forming an individual residence, typically on one floor and within a larger building containing a number of such residences, hence Option A is the correct answer, as a collection of individual residences or apartments are called flats.
Option B is incorrect because “congruence” means agreement or harmony, or compatibility which is incorrect.
Option C is incorrect because “architecture” means the art or practice of designing and constructing buildings, hence it is not the same as a collective name for a group of apartments.
Option D is incorrect because “condominium” means a group of many blocks or flats together, hence it is incorrect.