Step 1: Understanding the Concept:
Chronology in ancient Indian epics is determined by both tradition (Yuga system) and internal literary evidence (inter-textual references). Traditionally, the Ramayana took place in the Treta Yuga, and the Mahabharata occurred at the end of the Dvapara Yuga.
Step 2: Detailed Explanation:
Assertion (A) aligns with both the traditional and the modern scholarly view. The Ramayana is called the Adi-Kavya (the first poem), implying its priority in the literary timeline. The social structure in the Ramayana (focused on Ayodhya and Janakapuri) appears more archaic compared to the complex political landscape of the Mahabharata.
Reason (R) provides the critical internal evidence for this chronology. In the Vana Parva of the Mahabharata, the sage Markandeya narrates the Ramopakhyana to the Pandavas to console them during their exile. This proves that the story of Rama was already a well-established ancient legend by the time of the Mahabharata. Furthermore, Hanuman (a major character of the Ramayana) appears in the Mahabharata to meet Bhima. Conversely, in the Valmiki Ramayana, there is absolutely no mention of the Pandavas, the Kauravas, or the great war of Kurukshetra.
If the Mahabharata had been earlier, a text as comprehensive as the Ramayana would likely have made some allusion to it. The presence of the Ramayana's plot within the Mahabharata, and the absence of the Mahabharata's plot in the Ramayana, is a decisive literary proof that the Ramayana is the older text. Therefore, the Reason (R) perfectly explains and justifies the Assertion (A).
Step 3: Final Answer:
The assertion is true, and the reasoning based on inter-textual references is the standard logical proof for the priority of the Ramayana.