Step 1: Understanding the Concept:
The concept of the "Fifth Veda" (Panchamo Vedah) originates from the idea that while the four Vedas (Rig, Yaju, Sama, Atharva) are the ultimate authority, they are difficult for common people to study and understand. Therefore, a text that conveys the same Vedic truths (Dharma, Satya, Moksha) through stories and history is elevated to the status of a 'Veda' to show its equal spiritual importance.
Step 2: Detailed Explanation:
The Mahabharata, written by Veda Vyasa, is an encyclopedic epic. It famously claims about itself: "Yadihāsti tadanatra yannehāsti na tatkvacit" (Whatever is here, may be found elsewhere; but what is not here, is nowhere else). It contains the Bhagavad Gita, the Shanti Parva (treatise on statecraft and Moksha), and the stories of thousands of characters that illustrate the complexities of Dharma.
The designation of "Fifth Veda" for the Mahabharata is found in various Puranas and the Upanishads (like the Chandogya Upanishad, which refers to 'Itihasa-Puranam Panchamam Vedanam'). However, the Mahabharata is the primary text associated with this title because it was intended to bring Vedic wisdom to the 'Shudras and Women' (those who were traditionally restricted from direct Veda study) through the medium of history (Itihasa).
While the Ramayana (Option C) is equally holy, it is generally called the 'Adi-Kavya' (first poem). The Smritis (Options A and B) are derivative works and are never called the 'Fifth Veda'. The Mahabharata's sheer size (100,000 verses) and its exhaustive treatment of the four Purusharthas solidify its unique title.
Step 3: Final Answer:
The Mahabharata is the work recognized as the Fifth Veda.