It is given that there are 20 lunchboxes, 10 of the lunches have peanut butter cookies, 5 have snicker doodle cookies, and the last 5 boxes have chocolate chip.
The formula that the math teacher used is:

Here event A is getting chocolate chip cookies, n(A)=5 is the total number of cases with chocolate chip cookies and n(S) is total number of lunchboxes so it follows:

Now 1/4 is 25% and the formula is of theorotical probability.
Option D is correct.