There is no single entity responsible for ensuring the rules of the protocol are not violated. Rather, bitcoin is a decentralized network, so all participants must adhere to the protocol’s rules, or else the network will ignore them.
This is accomplished by a network of full nodes, that relay and validate all transactions and blocks against the network’s rules. All of the nodes together form the decentralized network, so to join the network you must adhere to the rules defined by the node’s code.
- The market forces behind the supply and demand of bitcoin
This will determine the price, but the price will not affect the ability of participants to violate the networ’s rules, nor will it ensure that rules are not violated. The rules must be followed whether the price is $1 per bitcoin, or $1 million per bitcoin.
Miners work to secure the blockchain record, they will validate transactions in the same way other (non-mining) full nodes do, and they will ensure only valid transactions are added to the blockchain.
- Consensus of the miners
Same as above. Note that even if a group of miners decided to violate the network’s rules, then the network (full nodes) would simply ignore any new (invalid) blocks found by those miners.
- Developers of the bitcoin protocol
The developers collaboratively write and refine the code that runs the network, but this does not give them absolute powers to violate or change the rules. Their code works to ensure that everyone involved in the network is behaving, and that any misbehaving nodes are banned.