Step 1: Understanding the Concept:
As discussed in previous questions, the Panchalakshana (Sarga, Pratisarga, Vamsha, Manvantara, Vamshanucharita) defines the traditional scope of a Purana. Among these, Manvantara deals with the cyclic nature of cosmic time.
Step 2: Detailed Explanation:
Assertion (A) is factually correct according to the standard definitions of Purana in the Amarakosha and various Mahapuranas like the Vishnu Purana. Manvantara is the fourth characteristic.
Reason (R) explains what the 'Manvantara' section of a Purana actually contains. A Manvantara is a period of time ruled by a Manu. According to Puranic chronology, there are fourteen Manus in one day of Brahma (Kalpa). For each Manvantara, the Puranas describe:
1. The name of the Manu (e.g., Svayambhuva, Vaivasvata).
2. The Sapta-rishis (seven sages) of that period.
3. The Indra of that period.
4. The names of the Manu's sons and his dynasty.
Since the description of these names and cycles is precisely what constitutes the topic of 'Manvantara', the Reason (R) is a perfect definition and explanation of why Manvantara is considered a distinct thematic component (A) of the Puranic literature. Without the genealogy of Manus and the cycles of time, the Purana would not fulfill its 'Panchalakshana' requirement.
Step 3: Final Answer:
Statement (A) is a categorical fact, and (R) provides the definition/description of that category. Hence, (R) explains (A).