Option A is Incorrect: ONDC does not mandate government control over transactions; rather, it promotes a decentralized, open market framework.

Option B is Incorrect: It does not seek to replace private e-commerce players. Instead, private applications and platforms integrate into the ONDC network to facilitate transactions.

Option C is Correct: The primary goal of the Open Network for Digital Commerce (ONDC) is to democratize digital commerce by shifting the industry from a closed, platform-centric model (dominated by a few large e-commerce giants) to an open, interoperable network. By enabling interoperability, ONDC breaks the monopolies of large private e-commerce players and creates a level playing field for small businesses, local retailers, and MSMEs to reach consumers directly.

Option D is Incorrect: ONDC does not mandate UPI for all online transactions. While it is frequently called the "UPI of e-commerce" because it applies the same open-network philosophy to online shopping, it does not enforce UPI as the sole payment method.

Therefore, the correct option is C.