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Correct answer: B. Only when a backwards-incompatible change is made, not on every update to the protocol.
Explanation
The principle — The version identifies compatibility, not recency.
Why the key is correct — The Model Context Protocol uses string-based version identifiers following the format YYYY-MM-DD, to indicate the last date backwards incompatible changes were made. And the corollary is stated: the protocol version will not be incremented when the protocol is updated, as long as the changes maintain backwards compatibility.
Why the others are wrong — Compatible updates do not move it, there is no hidden numeric version, and a new date does not automatically strand existing implementations.
Remember this — The date is the last time something broke.
Sources — MCP first-party documentation.
Sources
“The Model Context Protocol uses string-based version identifiers following the format YYYY-MM-DD, to indicate the last date backwards incompatible changes were made.”
“The protocol version will not be incremented when the protocol is updated, as long as the changes maintain backwards compatibility.”
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