In this task, you need to count the number of times the given letter appears in the given sentence.

Sentence: 'a police officer who is riding on a horse'. Find frequency of the letter 'p'
1

Sentence: 'a baseball player dropping his bat and starting to run'. Find frequency of the letter 'o'
2

Sentence: 'a person riding skis on a snowy surface'. Find frequency of the letter 'n'

4