Your answer would be c, that is, of course. The connecting in A "in other words" is used to explain something that has already been said by using other words for the listener/reader to understand. Then, "in fact", usually used in front position in a clause, is used to give more detailed information about what you have previously said. On the contrary, "furthermore", a more formal connective, is used to add information.
"Of course" is your answer because the other connectives do not apply and also because it is used to say that something is normal and shoud not surprise the person you are talking to. That is, the other words would make the sentence be semantically odd as it is shown in the following examples:
#Everyone appeared to be having a good time. In other words, my impressions may have been wrong
You are not re-explaining what you have said by using other words.
#Everyone appeared to be having a good time. In fact, my impressions may have been wrong
You are not adding more detailed information about what you have said.
# Everyone appeared to be having a good time. Furthermore, my impressions may have been wrong
You are not adding information about what you have said. The second clause has nothing to do with what you said in the first one.
