Assuming no routing loop is present, which situation would primarily cause a…

A dynamic routing protocol compares the candidate paths it has learned using its own metric and selects whichever candidate has the lowest value of that metric, trusting that value as “best” regardless of whether the number actually reflects real-world path quality. Different protocols compute this metric on different scales: RIP simply counts hops, OSPF derives a cost from link bandwidth, and EIGRP combines bandwidth and delay into a composite value — so a metric value computed by one protocol is not comparable to a metric value computed by another.

When routes learned from one protocol are redistributed into a different protocol, the receiving protocol has no way to translate the original metric onto its own scale, so it must instead assign a seed (default) metric to the redistributed routes. Because that seed metric does not represent the routes' true cost, a router can end up preferring a path that its own metric marks as best even though a different, non-redistributed path is actually shorter or faster — and this happens purely from the metric mismatch, with every protocol involved still converging normally and operating exactly as designed.

The other three situations, by contrast, each describe an actual breakdown in protocol operation rather than a side effect of protocols working correctly:

• A route-redistribution loop — the case this stem's “no routing loop” premise explicitly rules out — happens when a route is fed back into the very protocol it originated from; the metric keeps inflating (or the route oscillates) until it is discarded, a misconfiguration that breaks correct operation, not an artifact of it.

• An MTU mismatch on a link used by a link-state protocol typically prevents the two routers from ever completing neighbor/adjacency formation, so the protocol is not functioning correctly once that happens — it simply fails to form a relationship on that link.

• If link-state packets are never acknowledged, the reliable-flooding mechanism that keeps every router's link-state database synchronized has itself broken down — again a functional failure, not a consequence of correct operation.

So the situation that produces a suboptimal path while every routing protocol continues to run and converge correctly is unequal cost metrics between heterogeneous routing protocols.