The first letter of a sentence should be capital. So, 'W' in what is capitalised. This sentence expresses a sudden expression which tells that it was a pleasant day. Thus, it is an exclamation. So, the exclamation mark (!) will come at the end. Thus, option D is correct.
Option A is incorrect as it ends with a full stop and this isn't an assertive sentence.
Option B is incorrect because P and D in pleasant and day are capitalised respectively. Whereas, pleasant is an adjective and day is a common noun. So, they shouldn't be capitalised. Thus, option B is incorrect.