The plot structure intrigues readers because it takes place over a long period of time.
        
                    
             
        
        
        
The correct answer is D. Cannot stand as a sentence.
Subordinate clauses convey no meaning when left alone and must always be used in combination with independent clauses so as to create a complex sentence that conveys meaning.
        
             
        
        
        
He needs Brutus to support his plot against Caesar
        
             
        
        
        
No, it is not a correct sentence.
The mistake is in the word <em />worse - you need to use the superlative (final) form of the adjective. Bad - worse - worst. So the correct answer would actually be:
Deciding to go to Barcelona was the worst decision I ever made.