One of the most fundamental techniques of surviving in the harsh environs of a desert is
to drink as much water as you can.
(A) most fundamental techniques of surviving in the harsh environs of a desert is to drink as much water as you can.
(B) fundamental techniques of surviving the harsh environs of a desert is drinking as much water as possible.(C) fundamental techniques of surviving in the harsh environs of a desert is to drink as much water as you can.
(D) most fundamental techniques which can be employed to survive in the harsh environs of a desert is to drink as much water as is possible.
(E) fundamental techniques of survival in the harsh environs of a desert is
the drinking of as much water as one can.
To+verb, an infinitive can not be used to show an action as a gerund can be.
to drink needs to be replaced with 'drinking' to intend to mean the action of drinking and not the intent/objective to drink.
Between B and E
E has awkward construction, ''
the drinking of as much water as one can.'' doesn't seem logical or meaningful.