Final answer:
According to the adult Rule of Nines, the upper extremities account for 18% of the body surface area.
Step-by-step explanation:
The adult Rule of Nines is a method used to estimate the percentage of body surface area affected by burns. According to this rule, the upper extremities (arms) account for 9% each, totaling 18% of the body surface area. Therefore, the correct answer is option a) 18% each.