Bar charts compare separate items
A bar chart is best when the categories are distinct and there is no natural order to connect: sales by region, votes by candidate, counts by category. The length of each bar is the whole story, and the gaps between bars signal that the items are separate.
Line charts show change
A line chart is best when the points form a sequence — most often time. The connecting line implies continuity: what happened at 2 follows from what happened at 1. That is why stock prices, temperatures, and populations are drawn as lines, and why Extraplot asks you to extend a line rather than fill a bar.
Why it matters for guessing
When you see a line, you are being asked about what comes next in a sequence, so your guess should follow the existing path. If the same data were bars, you would be comparing unrelated values instead of predicting a continuation. Recognizing the chart type tells you which question is actually being asked.