Step 1: Understanding the Concept:
This question asks for the original or birth name of Mahavir, the 24th and last Tirthankara of Jainism.
Step 2: Detailed Explanation:
Mahavir was born into a royal Kshatriya family in ancient India. His childhood name was Vardhman, which means "one who grows".
He was given the name Mahavir (meaning "Great Hero") after he attained Kevala Jnana (omniscience).
Let's analyze the other options:
Gautama and Siddhartha: These are names associated with Gautama Buddha, the founder of Buddhism. His birth name was Siddhartha Gautama.
Tathagata: This is a title for the Buddha, meaning "one who has thus gone" or "one who has thus come".
Step 3: Final Answer:
Based on historical records, the real name of Mahavir was Vardhman. Therefore, option (A) is the correct choice.