Which routing protocol introduced variable length subnet masks?

RIP version 2 (RIPv2) introduced support for variable length subnet masks (VLSM), which allows for more efficient use of IP address space by enabling different subnets to have varying lengths. This feature enhances the flexibility of subnetting, as it accommodates scenarios where a single contiguous block of IP addresses can't simply be divided into equal-sized subnets.

The introduction of VLSM in RIPv2 allows for better address summarization and more efficient routing information exchange. This was a significant improvement over RIP version 1, which only supported fixed-length subnets and would not provide the capability to communicate varying subnet masks in routing updates. As a result, RIPv2 is better suited for modern networks that require more granularity in subnetting, making it the correct choice in this context.

The other protocols mentioned, such as IGRP and EIGRP, have their own capabilities and improvements over standard RIP, but it is specifically RIPv2 that is known for introducing support for variable-length subnet masks in the context of distance vector routing protocols.