Question

which is the single variable of frequency distribution

Answer 1

That is, a frequency distribution tells how frequencies are distributed over values.
Frequency distributions are mostly used for summarizing categorical variables. That's because metric variables tend to have many distinct values. These result in huge tables and charts that don't give insight into your data. In this case, histograms are the way to go as they visualize frequencies for intervals of values rather than each distinct value.
Anyway. Let's look at some examples of frequency distributions.