Block formatting is a type of letter format used which consists of aligning all the elements of the letter (Date, Letterhead, Salutation etc.) to the left margin of the page.
Modified block formatting is another type of letter format used, which consists of aligning all the elements of the letter to the left margin, except, the date, the sign off and the signature line, which are centered. Another characteristic is that each paragraph is indented 5 spaces.
Mixed punctuation is a style of punctuation that could be considered more "proper" and not as liberal because it requires a colon after the opening salutation and a comma following the complementary close.
Open punctuation is another style of punctuation that is more liberal because the colon and comma can be omitted.
In the text, it is mentioned that the letter is aligned to the left and that the opening salutation has a colon at the end, therefore, the correct answer would be option A: "block formatting with mixed punctuation".
Option B is incorrect because although the letter does have block formatting it does not have open punctuation. Option C and D are incorrect because the letter does not have modified block formatting.