A.
self-deprivation or
adopting an extreme diet, but
"self-deprivation" and "adopting" are not parallel.
INCORRECT_____________________________________________________________________________________
B. self-deprivation or the adoption of an extreme diet, but of
Correct idiom usage and correct parallelism in the list.
CORRECT_____________________________________________________________________________________
C. self-deprivation or the adoption of an extreme diet, but
Parallelism error between "self-deprivation" and "adoption" is corrected but the sentence should end with "of" to maintain the correct usage "not of X but of Y"
INCORRECT_____________________________________________________________________________________
D. depriving oneself or adopting an extreme diet, but
Idiom usage should be of the form "not of X but of Y"
INCORRECT _____________________________________________________________________________________
E.
depriving oneself or adopting an extreme diet, but that of
"depriving oneself" is not parallel with "healthy life style"
INCORRECT _____________________________________________________________________________________
Hence option B