"How many chairs do we put at the table?"
Translation:Quante sedie mettiamo a tavola?
From my understanding, though both tavola and tavolo mean table, tavola is more like "the place we eat" while tavolo is the table itself. Here's an article about it: http://blogs.transparent.com/italian/tavolo-or-tavola/
In another section a native Italian speaker said that they can be used interchangeably. Apparently seggiola is only used in a few areas of the Italy and sedia would be more widely used, but one of the initial authors of this section of DL was from the part of Italy that uses seggiola so it is used heavily in the suggested answers
Since tavola is the only choice, the sentence is referring to action around a meal; the definition of tavola is a table for meals. Tavolo is a piece of furniture. You eat at a tavola; you book a tavolo. Since there is no guidance in the lessons, the only clues you have are discerning how a word is used and in what context.