Given that the oracle and the "request/response" mechanism may be operated by multiple entities, the possibility of data being tampered with during transmission obviously exists. I want to know how chainlink guarantees the authenticity and accuracy of the data it returns to the smart contract.
The basic request model would never be something you go to production with, for exactly the reason described.
This is why you want to work with a decentralized Chainlink network. The same reason that blockchain is secure, makes this system secure. If 1 ETH node is hacked, no one cares because it's decentralized. If 1 Chainlink node is hacked no one cares, because it's decentralized.
Have multiple Chainlink nodes respond means that any outliers are rejected. They are highly financially inclined to respond accurately, otherwise they will not make any LINK. The data is brought on from many different resources to get a high quality decentralized aggregate.