Final answer:
The normal adult reaction to the Babinski test is no response.
Step-by-step explanation:
The normal adult reaction to the Babinski test is Option C: No response. In a normal adult, the plantar reflex causes curling of the toes and a moderate plantar flexion, rather than extension of the big toe and fanning of the other toes (Option A) or flexion of the toes and withdrawal of the foot (Option B). Pain at the stimulation site (Option D) is not an expected response to the Babinski test.