Parenthesis is used to include additional information in a sentence. It's also used in numbered or lettered lists, the parenthesis encloses the numbers or letters. Like in the given sentence, it encloses the numbers this way (1), (2), (3), (4). Hence option A is correct. The punctuation mark ':' is a colon mark that precedes a list, to indicate that a list will follow. ',' is a comma that indicates a pause in a sentence or to separate parts of a sentence, like in the given sentence it separates the parts "mark sheet", "leaving certificate", "birth certificate" and "reference letter". '.' is a full stop which is used at the end of a sentence, the sentence ends at "letter", so the full stop follows it. Since none of these punctuation marks are parentheses, options A, C and D are incorrect.