The answer is: (B) Magnificent should be written, "Magnificent!"
When citing what someone else has said, quotation marks are used. In this case, the expression takes an exclamation mark, which should be placed within the quotation marks.
The rest of the options are incorrect because it is proper to introduce quotation marks when citing, in choice C the exclamation mark is outside the quotation marks, and the use of semicolon is unnecessary to introduce a quotation.