The correct option is
A What a
The given sentence is exclamatory, and requires a suitable pronoun that refers to the noun 'person'.
Options A and D: 'What' and 'what a' are both exclamatory pronouns, that refer to a person or thing. While referring to singular nouns, like 'charming person', we use 'what a'. While referring to plural or uncountable nouns, like 'people' or 'advice', we use 'what'.
Since the noun here is singular 'person', 'what a' is the preferred option. Hence D is correct and A is incorrect.
Option B: 'How' refers to manner, and does not refer to the person.
Option C: 'That' isn't an exclamatory pronoun and hence not a suitable option.'
Thus, options A, B, C are incorrect.