The correct answer is D. in.
The word “deficient” means “lacking in something essential.” In this case, the diet is lacking in proteins and fats. Proteins and fats are essential nutrients that the body needs to function properly. They are used for building and repairing tissues, producing hormones, and providing energy.
The word “of” is used to indicate possession or association. For example, “the color of the sky” or “the owner of the car.” The word “on” is used to indicate location or position. For example, “the book on the table” or “the city on the hill.” The word “for” is used to indicate a purpose or goal. For example, “the food is for the children” or “the medicine is for the patient.”
In the sentence “Your diet is deficient in proteins and fats,” the word “in” is used to indicate that the diet is lacking in these nutrients. The diet is not lacking in proteins or fats because of its location or position, or because it is not intended for someone who needs these nutrients. It is lacking in these nutrients because it does not contain enough of them.