Option C: The given sentence is interrogative as it begins with the question word 'why' and intends to ask a question. Hence the correctly punctuated form is seen in option C, as it begins with a capital letter 'W' and ends with a question mark '?'.
Option A: A comma is used to separate clauses and phrases. A comma also separates words (from other words or the sentence on the whole) that are not essential to the meaning of the sentence. The given sentence does not require a comma, because "to school on Monday" is an essential part, without which the meaning of the sentence is incomplete. Hence A is incorrect.
Option B: The error lies in the capitilization of school: 'School' and the exclamation mark '!'. 'School' is a common noun and does not begin with a capital letter.
The given sentence is interrogative (?) and not exclamatory (!). Exclamatory sentences express shock, surprise or wish. Hence B is incorrect.
Option D: Declarative or imperative sentences end with a full stop '.' Such sentences declare something or make a request or command. They do not ask questions. Hence option D is not punctuated correctly.