Vessels regularly navigating Ohio and Mississippi rivers above Cairo, Illinois, and their tributaries, shall at all times have on board a copy of __________.

Correct Answer: Option B — U.S. Army Corps of Engineers Navigation Regulations (Blue Book)

Vessels navigating the Ohio and Mississippi rivers above Cairo, Illinois, must comply with specific U.S. Army Corps of Engineers regulations. As mandated by 33 CFR, these operators must maintain an onboard copy of the U.S. Army Corps of Engineers Navigation Regulations, commonly known as the Blue Book.