Final answer:
Mixed punctuation in letters refers to using a colon after the salutation and a comma after the complimentary close in formal correspondence.
Step-by-step explanation:
Mixed punctuation in letters refers to the use of multiple punctuation marks that follow the salutation and complimentary close in a business letter or formal correspondence. The standard format for this is to use a colon after the salutation (e.g., 'Dear Sir/Madam:') and a comma after the complimentary close (e.g., 'Yours sincerely,'). Therefore, the correct answer to which of the following describes mixed punctuation in letters is 1) The use of both a colon and a comma in a letter. Mixed punctuation is distinct from open punctuation, where these punctuation marks are omitted.