In Chi-Square Tests, There Is a Rule Known as Rule

In chi-square tests, there is a rule known as Rule of Five. What does this rule require?

A) It requires that the observed frequency for each cell be five or more.
B) It requires that the degrees of freedom for the test be at least five.
C) It requires that the expected frequency for each cell be five or more.
D) It requires that the difference between the observed and expected frequency for each cell be at least five.