Step 1: Understanding the Concept:
A recluse is a person who lives a solitary life and avoids other people.
Step 2: Detailed Explanation:
"Ascetic" practices severe self-discipline, often for religious reasons — may live alone but not necessarily avoids people. "Recluse" specifically means one who withdraws from society. "Unsocial" is an adjective, not a noun. "Agnostic" relates to religious belief (unsure about God's existence).
Step 3: Final Answer:
Thus, "Recluse" is the correct substitution.