The correct answer is: Matrix structure.
Matrix structure is the only alternative that would be sufficiently flexible (without the rigidity of bureaucracy) but relatively stable and co-ordinated (unlike ad hocracy).
The organisation is said to use project teams - which should suggest matrix management. It is too large for a simple structure (centralised control by a single leader).
The terms 'ad hocracy', 'simple structure' and 'professional structure' come from Mintzberg's classification of organisation types.
