What is a soft fork in Bitcoin?

A soft fork in Bitcoin refers to a backward-compatible change to the protocol that allows for new rules to be introduced without requiring all users to upgrade their software. This means that nodes that do not upgrade can still recognize and follow the rules of the existing protocol, ensuring compatibility with those that adopt the new changes.

In the context of Bitcoin, soft forks typically enable the implementation of new features or enhancements, such as the introduction of new address types or changing the way transactions are validated, without disrupting the overall functioning of the network. Because it's a backward-compatible change, older versions of the software can still operate alongside the new rules without any loss of functionality or the threat of network fragmentation.

This is distinct from a hard fork, where changes are not compatible with previous versions, potentially leading to two separate versions of the blockchain if all users do not upgrade. Additionally, not all upgrades or changes to the network necessitate that every participant adopts them immediately, giving flexibility to miners and users alike. Thus, the correct choice reflects the fundamental feature of a soft fork in that it allows for upgrades without mandating universal compliance among all network participants.