It's asking "what are the set of y coordinate values for all of the points on the red line?"
In this case, all of the points on this red line have a y coordinate of y = 6. Two such points on this red line are (0,6) and (1,6)
So that's why the only answer is choice D) 6. This is the only number in the range. The range is the set { 6 }. No other number is in the range, so no other choice A, B, C, E, F are true
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Answer: Choice D) 6
(note: none of the other boxes are checked off)