The reason that percentages are used for the histogram is given by:
Percentages account for population size, so they allow for direct comparisons between groups of varying sizes.
An histogram is a graph that shows the number of times each element of x was observed.
When different sized measures are compared, percentages are recommended, and one example is the age of the mother of first born children in this problem, as the US has a large population, hence all bins will have large absolute numbers, greater than other countries with smaller populations, but the percentages will be a better measure to see for example how young the mothers are when they have their first children.
Hence the correct option is:
Percentages account for population size, so they allow for direct comparisons between groups of varying sizes.
More can be learned about histograms at https://brainly.com/question/28164315
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