Which chart element provides the boundaries of the graphic?
a. legend
b. chart area
c. slices
d. chart elements

The correct option is B.
Chart area is the boundary that contains all the chart and all its elements including the chart titles, legends, plot area, labels, etc.
The legend is the key that identifies the gradient, texture, colour and pattern fill given to each data series. 
The chart title is the label that describe the chart while the plot area is the section of the chart that contains graphical representation of the values in a data series.

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