Answer:
A. by alluding to the visitor as being Herbert.
Step-by-step explanation:
In this passage, we learn about the scraping of the chair, the knocks that echo through the house and the difficulties that Mr. White goes through in order to find the paw. However, what the author does not suggest is that the visitor is Herbert. In fact, in the only instance in which the author refers to the knock, he says that it was "the thing" that wanted to come in. Therefore, this is not one way in which the author builds suspense.