Answer:
C. 10
Explanation:
You want to know the bin size in a histogram with bin boundaries 10.5, 20.5, 30.5, ....
Bin size
The size of the bin is the difference between its boundary values. Here, all of the bins are the same size:
20.5 -10.5 = 10
The bin size is 10.
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Additional comment
The bin boundaries are typically chosen so that no data value can fall on the boundary, but must be unambiguously allocated to one bin or another. When the data values are integers, it is not uncommon to have the bin boundary be halfway between two integers.
Here, the boundary being 20.5 means a score of 20 will be counted in the bin to the left, and 21 will be counted in the bin to the right.
The boundary value shown at the right edge of the graph is not 100.5, as we might expect. However, we can reasonably assume that a test score of 100 would be counted in the bin with upper boundary 100, without any ambiguity.