For Bitcoin, transactions are confirmed by inclusion in blocks, and the network targets an average block interval of about 10 minutes. That means transactions are ''added'' to the Bitcoin blockchain approximately every 10 minutes in the sense that a new block containing a batch of transactions is appended at that cadence. The 10-minute target is achieved by a difficulty adjustment mechanism that recalibrates mining difficulty roughly every 2016 blocks, aiming to keep the average interval stable despite changes in total network hash power. It is important to note that this is an average: blocks can be found faster or slower in the short term due to the probabilistic nature of proof-of-work mining. Other blockchains have different block times (seconds to minutes), but the question's options and typical curriculum context align with Bitcoin's 10-minute design. Therefore, the correct choice is approximately every 10 minutes.