Which sentence uses a conjunctive adverb? "They wanted to go to New England this summer; therefore, they've saved for a whole year."

Or

"They wanted to go to New England this summer, so they've saved for a whole year."

"They wanted to go to New England this summer; therefore, they've saved for a whole year."

"Therefore" is the conjunctive adverb; it connects the two independent clauses.

Conjunctive adverbs are used to show sequence, contrast, cause and effect, and other relationships.

Notice the punctuation: there is a semi-colon before it and comma after it.

Other examples of such adverbs are ... otherwise, nevertheless, however, meanwhile, etc ...

"So" (in the other sentence option) is a coordinating conjunction that (also) serves to connect the two independent clauses. Notice that only a comma is used after it.

Other examples are ... and, but, yet, or, for, nor.