The correct answer is C. Subcutaneous fat.
To avoid injury to the temporal branch of the facial nerve in this area, undermining should occur in the subcutaneous fat layer. Undermining deeper than the subcutaneous fat layer (SMAS or muscle) could risk injury to the temporal branch of the facial nerve.
Bolognia J, Jorizzo J, Schaffer J, editors. Dermatology, 3rd ed. Philadelphia: Elsevier Saunders; 2012.