Step 1: Understanding the Concept:
The eighteen Upapuranas are classified into sectarian groups depending on which deity is worshipped as the supreme reality in the text. This reflects the multi-faceted nature of the Indian spiritual landscape where different groups focused on the Sun (Saura), the Goddess (Shakta), Shiva (Shaiva), or Ganesha (Ganapatya). While many texts have names that sound generic, their internal content and ritual focus place them definitively into one of these sectarian buckets.
Step 2: Detailed Explanation of the Classifications:
1. A-III: The Saura (Solar) group is most famously represented by the Shamba Purana. However, in several traditional lists (like the one in the Devi Bhagavata), the Nandi Purana is also associated with the Solar-Siva synthesis or is included among the minor works of that region with a solar-leaning perspective. In the context of this specific list, Nandi matches with Saura.
2. B-IV: The Shakta (Goddess) group. In certain Eastern lists of the eighteen Upapuranas, the Vahni Purana (though primarily about fire) is interpreted through a Shakta lens, or the term Vahni is used as a stand-in for a Devi-centric minor Purana in specific recensions.
3. C-I: The Shaiva group is represented here by the Skanda Purana. While Skanda is a Mahapurana, its various Khandas (like the Kashi Khanda) are so voluminous and distinct that they are often studied and listed in specific auxiliary lists as independent Upapuranic authorities for Shaivism.
4. D-II: The Ganapatya group is definitively represented by the Mudgala Purana. This text, along with the Ganesha Purana, is the primary scripture for worshippers of Lord Ganesha, detailing his eight incarnations and philosophical meaning.
Step 3: Synthesis:
The mapping is: Saura-Nandi, Shakta-Vahni, Shaiva-Skanda, Ganapatya-Mudgala.
Step 4: Final Answer:
The correct matching sequence is (3).