Final answer:
The sign used to mark the end of guardrails of a narrow bridge or other obstruction is the End of road sign.
Step-by-step explanation:
The sign used to mark the end of guardrails of a narrow bridge or other obstruction is the End of road sign. This sign indicates that the road or path is coming to an end and drivers should proceed with caution. It is important to be aware of this sign and adjust your driving accordingly to ensure safety.