Answer:
The correct answer would be A. gulf. 
We can immediately eliminate option B and D, because rivers and straits are normally smaller than bays. 
Option C wouldn't make sense either, since an estuary is not an area enclosed by land, but rather an area where streams and rivers flow into a larger body of water, like an ocean.
Ultimately, this gives the correct answer to be A. Gulf. 
Let me know if this helps!