Step 1: Understanding the Concept:
This question deals with the classification of Puranic literature. While Ganesha is a universally worshipped deity, the texts dedicated primarily to Him (Ganesha Purana and Mudgala Purana) have a specific historical and canonical placement.
Step 2: Detailed Explanation:
Assertion (A) is correct. The standard canonical list of the 18 Mahapuranas was established relatively early and is preserved in almost all major Puranas (like the Matsya or Vishnu). This list includes Puranas dedicated to Brahma, Vishnu, Shiva, and some to Agni or Surya, but it does not contain a Ganesha Purana. Therefore, when Ganesha-centric sectarian literature emerged later, it was categorized under the Upapuranas.
Reason (R) explains the logic behind this categorization. The 18 Mahapuranas represent the "Mahakavyas" of the Puranic world, largely balanced between the three main deities (Trimurti). Although Lord Ganesha is mentioned in the Mahapuranas (like the Brahmavaivarta), he was not the 'primary hero' or the subject of an entire Mahapurana in the initial codification. As the Ganapatya sect (worshippers of Ganesha) gained prominence in later centuries (around the 10th-14th century), they composed texts like the Ganesha Purana. Since the list of 18 Mahapuranas was already 'closed', these newer, sectarian texts were naturally placed in the Upapurana category. Thus, the absence of Ganesha from the primary Mahapurana list is indeed the reason why His dedicated Purana is an Upapurana.
Step 3: Final Answer:
Both statements are correct. The fixed nature of the Mahapurana list (R) is the reason why Ganesha-centric texts (A) are classified as Upapuranas.