Answer :
Option B. On hot days, they say to drink plenty water. (The sentence makes no sense. So it is incorrect.)
B. On hot days, they say to drink plenty of water.
This sentence is incorrect. It should be:
On hot days, you should drink plenty of water
OR
Plenty of water should be drunk on hot days
OR
They say plenty of water should be drunk on hot days.
This sentence is incorrect. It should be:
On hot days, you should drink plenty of water
OR
Plenty of water should be drunk on hot days
OR
They say plenty of water should be drunk on hot days.