In English vocabulary and personality descriptions, what is the correct one word substitution for a person who remains indifferent to both pain and pleasure, staying calm and unemotional in every situation. Choose the best option.

Introduction / Context:
This question tests your knowledge of English words used to describe personality types and attitudes toward life. Many competitive exams include one word substitutions for such traits. Here, the description is "one who is indifferent to pain or pleasure", meaning someone who remains calm, controlled, and unaffected by emotional highs and lows.

Given Data / Assumptions:
- The person does not react strongly to pain or pleasure.
- The key idea is emotional control and indifference to external circumstances.
- We need a single word that names such a person.

Concept / Approach:
The word "Stoic" comes from the ancient philosophy of Stoicism, which taught that people should develop self control and not be disturbed by pleasure or pain. In modern English, a "Stoic" is someone who accepts suffering, difficulty, or joy without showing strong emotion. The other options describe different attitudes or types of people, not specifically this calm indifference to pain and pleasure.

Step-by-Step Solution:
Step 1: Focus on the phrase "indifferent to pain or pleasure". It suggests emotional balance and lack of outward reaction. Step 2: Recall that "Stoic" is used in sentences like "He remained stoic during the crisis", meaning he did not show fear or distress. Step 3: Examine "Eccentric", which means a person with odd or unusual behaviour, not necessarily calm under pain. Step 4: Consider "Philosopher". A philosopher is a thinker or lover of wisdom, which is a different idea. Step 5: "Fatalist" describes someone who believes that everything is predetermined by fate and cannot be changed. Such a person might or might not be calm. Step 6: "Optimist" is a person who looks at the bright side of life, expecting good outcomes. Step 7: Only "Stoic" directly matches the idea of remaining indifferent to both pain and pleasure.

Verification / Alternative check:
Checking dictionaries, "Stoic" is defined as "a person who can endure pain or hardship without showing their feelings or complaining". This is very close to the wording in the question. Quick reference books for one word substitutions also list "Stoic" as the answer for similar definitions. None of the other words carries this exact meaning in standard usage.

Why Other Options Are Wrong:
"Eccentric" is about unusual or odd behaviour, not emotional stability. "Philosopher" refers to a thinker, which may or may not describe the attitude to pain and pleasure. "Fatalist" is about belief in destiny, not necessarily about remaining calm. "Optimist" focuses on expecting good things, not on reacting calmly to pain or pleasure. Therefore, these options do not satisfy the definition.

Common Pitfalls:
A common mistake is to choose "Fatalist" because a fatalist might accept events as they happen. However, the question emphasizes indifference to pain and pleasure, not belief in fate. Another confusion is between "Stoic" and the adjective "Stoical"; both relate to the same concept, and exams might use either form. Remember that the noun for such a person is "Stoic".

Final Answer:
The correct one word substitution for a person indifferent to pain or pleasure is Stoic.