Final answer:
The city Huck and Jim missed as a landmark to turn down the Ohio River was Cairo. Missing this landmark, they continued on the Mississippi River, which moved them farther from their intended destination.
Step-by-step explanation:
The city that was a landmark to turn down the Ohio River that Huck and Jim missed in 'The Adventures of Huckleberry Finn' is Cairo. Cairo, Illinois, is situated at the confluence of the Ohio and Mississippi rivers and serves as a critical geographic marker for the characters in the novel. Missing Cairo meant that Huck and Jim continued down the Mississippi, further away from the freedom they sought in the Southern states during the pre-Civil War era.