Explanation:

When there are different number of observations in two histograms, the frequency of histogram-1 class interval might be high when compared to frequency of histogram-2 class interval but it might be because of more number of observations in histogram-1.

For example

There are 500 observations is first histogram and 50 is frequency of one class interval.

There are 100 observations is second histogram and 30 is frequency of same class interval.

Here if we compare frequency we say frequency of histogram-1 is high that of frequency of histogram-2 in that class interval. But in reality only 10% of data fall in that class interval in histogram-1 and 30% of data fall in that class interval in histogram-2

Hence when comparing two histograms, it is best to use relative frequency histograms rather than frequency histograms when the data distributions have different number of observations.

Option d is correct.

When there are different number of observations in two histograms, the frequency of histogram-1 class interval might be high when compared to frequency of histogram-2 class interval but it might be because of more number of observations in histogram-1.

For example

There are 500 observations is first histogram and 50 is frequency of one class interval.

There are 100 observations is second histogram and 30 is frequency of same class interval.

Here if we compare frequency we say frequency of histogram-1 is high that of frequency of histogram-2 in that class interval. But in reality only 10% of data fall in that class interval in histogram-1 and 30% of data fall in that class interval in histogram-2

Hence when comparing two histograms, it is best to use relative frequency histograms rather than frequency histograms when the data distributions have different number of observations.

Option d is correct.