Answer:
The sentence that shows correct use of an em dash is sentence A.
Step-by-step explanation:
The em dash is one of the most versatile punctuation marks. It can take the place of commas, parentheses, or colons. In this case, the em dash has been used instead of commas. This NP modifies the noun "the book". In spite of taking the place of commas, it is also important to bear in mind that dashes are always more empthatic than commas.