Choose the correct verb form: He tried to _____ the entire length of the hall by himself but had to call for assistance halfway down.

The correct choice, "walk," makes sense in the context of the sentence because it conveys a reasonable and typical action for someone attempting to traverse the entire length of a hall on foot. The use of "walk" suggests a steady, deliberate pace that can be maintained over a longer distance, reflecting the scenario of the individual starting the journey alone and eventually needing assistance.

While "skip," "sprint," and "run" might imply movement toward the end of the hall, they do not fit as well with the context provided. "Skip" suggests a form of playful movement that may not align with the idea of covering a long distance seriously. "Sprint" implies a quick, short burst of speed, which does not indicate sustained effort over a longer distance. "Run," while a valid way to cover distance, usually connotes a faster pace that might be less realistic given the context of calling for assistance halfway down the hall.

Using "walk" aligns with the imagery of someone attempting a long journey at a moderate pace, making it the most suitable choice.