A round yellow sign displaying a black “X” and the letters “RR” warns of:

Correct answer: C – A railroad crossing ahead
The round advance-warning sign tells drivers that a railroad crossing is ahead. Look both ways, listen for trains, yield when required, and be prepared to stop if a train or on-track equipment is approaching. The crossbuck is normally located at the crossing itself.
Source: Nebraska Driver's Manual — § 3B-2, “Sign Shapes”, Nebraska Driver's Manual — § 3B-4, “Warning Signs”


