Answer:
The correctly punctuated sentence is b. Three major cities -- Hartford, Bridgeport, and Stamford -- make up the crucial voting blocks of Connecticut.
Explanation:
An <u>em dash</u>, also called <u>long dash</u>, is used to mark off information or ideas that are not essential to undersand the rest of the sentence. They are used in pairs, surrounding the secondary information, in this case, <em>Hartford, Bridgeport, and Stamford.</em>