(A) CORRECTThis choice correctly sets off the two days, Sunday and Saturday, as parallel elements. The opening modifier modifies each of these separately: On most calendars, Sunday is the first day of the week… On most calendars, Saturday (is) the last (day of the week). The parallelism allows you to pick up the words is and day of the week and apply them equally to the second parallel element.
(B) Incorrect
Modifier: The second part of this choice, Saturday being the last, is now constructed as a modifier, but it does not logically refer to the text before it (Sunday is…). Rather, Saturday and Sunday should play equal roles in the sentence; one should not modify the other.
(C) Incorrect
Sentence Structure: This one is almost okay! However, it’s missing the word and between the two parallel elements. This is a proper sentence: The first day of the week is Sunday and the last is Saturday. This is a run-on sentence: The first day of the week is Sunday, the last is Saturday.
(D) Incorrect
Modifier: a comma –ing modifier is an adverbial modifier, not a noun modifier. It refers to the main subject and verb of the clause to which it is attached. In this case, that clause is Sunday is the first day of the week. It is illogical to say that Sunday is the first day ends on Saturday. The week ends on Saturday, not Sunday or the first day of the week.
(E) Incorrect
Meaning / Parallelism / Pronoun: This choice is trying to say that the week ends with Saturday, but the parallel structure implies that the subject pronoun it refers to the subject the first day. The first day does not, of course, end with Saturday.