I’m pretty sure that the answer is c
To improve a conclusion, a writer must bare in mind that it should not address a new topic, that is to say, it has to be a recompilation of the topics already mentioned and developed in the paragraphs, so it has to be a kind of "summary", an overall of the topics and ideas discussed. It should be concise and the reader needs to be know it is the conclusion and you are not going to keep on working on the ideas, so sometimes, it is necessary to include the words "to conclude"/ "to round up"/ "in conclusion" or something similar on order to let the reader know you are about to finish.
The sentence that contains a dangling modifier is C) Looking through the window, the room seemed very large.
It is dangling because we don't know who the participial phrase <em>looking through the window </em>refers to. It certainly doesn't refer to the room, the room wasn't looking through the window.
