We use an exclamation mark to indicate an exclamatory clause or expression in writing. ‘Wow’ is a commonly used exclamation that indicates surprise. Therefore it should be followed by an exclamation mark. Similarly, ‘What an expression she had’ is another sentence that indicates exclamation, therefore the first letter of the sentence needs to be capitalized and it needs to be followed by an exclamation mark, at the end. Hence option C is correct. A full stop is used in a declarative sentence and not in an exclamatory sentence. Hence options B and D are incorrect. Similarly, a question mark is used to punctuate an interrogative sentence that demands an answer and not an exclamatory sentence that indicates surprise. Thus option A is incorrect.