mark use to___along that river every weekend when he was young

The correct form to complete the sentence is "used to".

So the complete sentence is: "Mark used to fish along that river every weekend when he was young."

Explanation:
The phrase "used to" is employed to describe an action or situation that was true in the past but is no longer true in the present. It indicates a habitual action or a repeated event that occurred regularly.

In this case, the context suggests that Mark had a routine of fishing along the river every weekend during his youth. The use of "used to" highlights that this was a regular occurrence in his past, which contrasts with the present situation where he likely does not do this anymore. This structure is commonly used in English to express memories or past habits, making it suitable for this sentence.