Which consensus mechanism is primarily used by Ethereum to validate transactions?

Ethereum initially utilized Proof of Work (PoW) as its consensus mechanism to validate transactions. In this model, miners compete to solve complex mathematical puzzles in order to confirm transactions and add new blocks to the blockchain. This requires substantial computational power and energy consumption, as participants must invest in hardware and electricity to mine effectively. PoW was integral to Ethereum's functioning in its earlier days, ensuring the security and integrity of the network by requiring significant resources for potential attackers to tamper with transaction history.

It's important to note that Ethereum has since transitioned to Proof of Stake (PoS) with the Ethereum 2.0 upgrade, which allows validators to create new blocks and confirm transactions based on the number of coins they hold and are willing to "stake" as collateral. However, if the question refers to Ethereum's operations before this transition, PoW is the correct answer. This highlights the evolution of Ethereum's technology, as it aims to improve scalability and reduce energy consumption through a more environmentally friendly consensus method such as PoS.