Solution:(a) It is believed by my grandmother that drinking a glass of milk at bedtime is essential for health. (Correct)(b) It is being believed by my grandmother that drinking a glass of milk at bedtime is essential for health. (Incorrect use of 'beings' )
(c) It will have been believed by my grandmother that drinking a glass of milk at bedtime is essential for health. (Incorrect Tense)
(d) It has been believed by my grandmother that drinking a glass of milk at bedtime is essential for health. (Incorrect Tense)