The correct answer is (C) Agreement, harmony, or union in an undertaking, whether a design, plan, or joint action. Also, a musical performance, usually by several singers or musicians.

Concert comes from French concerter, Italian concerto, Latin concertare (decide together)

Examples of use:

• The worked and saved in concert to accumulate funds for a house.
• It took all persons acting in concert to erect the barn before the rains came.