The conspirators met __________ in order to plot a(n) __________ against the oppressive governance of Julius Caesar.

The term 'conspirators' has been used in the first part of the sentence. A conspirator is someone who plots a conspiracy i.e, secretly plot to overthrow the government. 'Clandestine' means to do something secretly. 'A referendum' is a term used to describe a plebiscite.Therefore, 'plot a plebiscite' is an incorrect expression. We can eliminate option A.

'Wittily' cannot be used to fill the first blank. The sentence talks about a serious event. Therefore, we can eliminate option C as well. 

Among met 'clandestinely' and 'wickedly', clandestinely is a better option since it captures the nature in which a conspiracy is carried out. Therefore, option B is the right answer.