Adiamond shaped sign with a measurement on it, such as 12' 6'' is most likely a sign.

The answer is W12-2. It is displayed as a diamond-shaped sign. It displays a black arrow pointing upward above the notation "12'-6", which is above a black arrow pointing downward. Low Clearance W12-2 signs shall be used to caution road users of clearances less than 12 inches directly above the statutory maximum vehicle height. In the situation of an arch or other building under which the clearance varies greatly, two or more signs should be used as required on the structure itself to give information as to the clearances over the entire roadway.

The answer is the third one or c