Although she was tired from the long trip, she insisted on helping with the preparations.
Correct: C
Option C correctly uses a comma to separate the introductory dependent clause ('Although she was tired from the long trip') from the main independent clause ('she insisted on helping with the preparations'). Option A lacks the necessary comma. Option B incorrectly uses a semicolon, which is not used to separate a dependent clause from an independent clause. Option D misplaces a comma, incorrectly breaking up the introductory clause.