Can a Miner Choose to Process a Transaction with a Zero Fee?

Yes, a miner can choose to include any valid transaction in a block they mine, including those with a zero fee. The miner is the one who constructs the block, so they have full discretion over which transactions from the mempool to prioritize.

While fees are the primary incentive, a miner might process a zero-fee transaction for altruistic reasons, for a private agreement, or if it is their own transaction. However, in a competitive mining environment, zero-fee transactions are usually ignored in favor of those offering a fee.