In reply to a.

If you prefer. It depends whether you are talking about the length of the sequences of Heads and Tails or whether you are talking about the number of sequences. In the first example, the length of each sequence is too long. In the second example, each sequence is too short. However, it is also true that the first example has too few sequences and the second example has too many. No matter how you look at it, the third example is "just right," which is the point of the article.