The sentence we are given is: 'THE LETTER ARRIVED EVERY MONDAY.'
The noun in this sentence, which can be replaced by a pronoun is 'letter'. The letter is in singular form, in plural form it will becomes letters. If you want to use pronoun to represent the word 'letters', the correct pronoun to use is 'they'.
Thus, the sentence will become: THEY ARRIVED EVERY MONDAY. This is the plural form of the sentence that is given in the question.