The y - axis subject for the graph would be 2015 Visitors ( millions).
The option that would be appropriate for the missing label would be B. National Parks
The national park that had approximately 5.5 million visitors would be B. Grand Canyon.
What does the graph show ?
The y-axis of a graph typically represents the variable that is being measured, in this case, the number of visitors to the top ten national parks in 2015. Since the numbers on the y-axis are likely in millions, the appropriate label is "2015 Visitors (millions)."
The missing label, which is likely the x-axis label, should denote what each bar in the bar graph represents. Since the bars are colored differently and seem to correspond to different national parks, the label "National Parks" would be the most appropriate.