Which of the following is a countable noun?

A count noun (or "countable noun") is a noun that can be counted, can be plural, and can be used with both singular and plural nouns. Headache, toothache, and stomachache are all count nouns because they meet those three criteria, as shown in the examples below.

If you keep complaining of stomachaches, I’m going to take you to the doctor. (stomachaches is plural)

I feel fine today, but I had two bad headaches this week. (two + headaches)

Toothaches are more common in this population. (toothaches + plural verb)

These “ache words” are more commonly used in their singular form, probably because people have only one ache at a time, but what matters is that these words do have a plural form and can be used with a plural verb.