Answer:
B. Ironic.
Step-by-step explanation:
Nathaniel Hawthorne's "Dr. Heidegger's Experiment" is about the story of how a scientist Dr. Heidegger worked on an experiment that tries to retain youth. This story works on the theme of the mistakes that people make when working against what had been designed by nature/ god. it also shows the greedy nature of men that leads to more destruction for them.
The sentence that is given in the question
"Mr. Gascoigne's mind seemed to run on political topics, but whether relating to the past, present, or future, could not easily be determined, since the same ideas and phrases have been in vogue these fifty years."
shows the ironic tone of the narrator when he said these. It is ironic in the sense that the very same "ideas and phrases" that are related "to the past, present or future" are still "in vogue these fifty years". The narrator did not specifically mention what era of politics was being discussed nor is there any indication of the time frame. But, the narrator still states the fact that "the same ideas and phrases have been in vogue these fifty years." The same things that were important in those days were still very much the same even now. By pointing out that Mr. Gascoigne's opinions are not determinable, he is implying that these things are still relevant today.