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

Sentence: 'a passenger bus that is driving down the street'. Find frequency of the letter 'r'
3

Sentence: 'a street scene looking at the back of the stop sign'. Find frequency of the letter 't'
6

Sentence: 'close up of various doughnuts including chocolate, sprinkled, green, striped, etc'. Find frequency of the letter 'u'

5