Answer:
To make the sentence correct, we must:
B. Add a comma after Shelley and another comma after century.
Step-by-step explanation:
In the sentence we are analyzing here, the structure "who lived in the nineteenth century" is an adjective clause. That means it functions just like an adjective, modifying a noun present in the sentence. In this case, the adjective clause refers to the proper noun(s) "Mary Shelley", and it is giving us further information about her.
When the purpose of the adjective clause is to explain, to provide further information, it should be framed by commas. That is the case here. For the sentence to be correctly punctuated, we must place a comma right after "Shelley" and another after "century", that is, a comma before the clause begins and another once it is finished:
Mary Shelley, who lived in the nineteenth century, was the author of Frankenstein.