Which version of RIP does not support Variable Length Subnet Masking (VLSM)?

The correct answer is B, RIPv1, because it is the original version of the Routing Information Protocol and operates using classful routing. Classful routing protocols do not carry subnet mask information in their routing updates, which means they cannot support Variable Length Subnet Masking (VLSM). This limitation prevents RIPv1 from efficiently handling networks that may use different subnet masks across the same major network class.

In contrast, RIPv2, which is an enhancement of RIPv1, includes subnet mask information in its routing updates, allowing it to support VLSM and classless inter-domain routing (CIDR). RIPng, which is designed for IPv6, also supports VLSM, as does RIPv2. Thus, the ability to work with variable-length subnet masks is a key distinction that sets RIPv1 apart from its successors.