Which sentence below is properly punctuated?
a. we plan to return some day to brazil; therefore, we want to visit rio, sao paulo, and manaus.
b. we plan to return some day to brazil: therefore, we want to visit rio, sao paulo, and manaus.
c. we plan to return some day to brazil; therefore, we want to visit rio; sao paulo; and manaus.

a.

Explanation:

The semicolon is correctly placed before the conjunctive adverb "therefore" that is being used here to join two independent clauses, correctly followed by a comma as conjunctive adverbs introducing the second clause in a sentence should be. Option c, while it follows this rule, is wrong because it uses semicolons instead of commas to separate the list elements (i.e. places that "we" want to visit).