Which warning sign would indicate that a driver should prepare for a bridge that may open?

The choice indicating "Opening or swing bridge ahead" is the correct response because it directly alerts drivers about the specific type of bridge that is designed to open for the passage of boats or larger vessels. This warning sign serves as a critical piece of information, indicating that drivers should be prepared for a possible stop or delay when approaching the bridge, as it may open unexpectedly.

The other options provide different types of warnings but do not specifically signify that a bridge will open. For example, "Bridge under repair" suggests maintenance work but doesn't indicate a movable bridge. "Bridge construction zone" alerts drivers to construction activities, which may cause delays but not necessarily open a bridge. Lastly, "Weight limit on bridge" warns about the maximum load allowed but has no relevance to whether the bridge will open for marine traffic. Thus, the sign about the opening or swing bridge is essential for ensuring safe and informed navigation near such structures.