Answer: B. Mary Shelley wants to write a great Gothic story, but she can’t think of an idea.
Step-by-step explanation: In the given excerpt from the introduction to "Frankenstein" by Mary Shelley, we can see how she describes her struggle to think of an idea to write a horror story compared to the ones she had read, but despite her efforts and constant thinking, she wasn't able to think of an idea of that she liked. So the correct option is B: Mary Shelley wants to write a great Gothic story, but she can’t think of an idea.