Explanation:

- Option 1: Double coincidence of wants

- This is directly related to the barter system.

- Both parties involved need to have what the other wants.

- A trade happens only when wants are mutually satisfactory.

- Correct

- Option 2: Common unit of value

- Not related to barter.

- Relates to money systems, where there's a standard measure of value.

- Option 3: Limited exchange

- Barter is often limited because of the need for a double coincidence.

- Challenging for large or complex trades.

- Correct

- Option 4: Both (a) and (c)

- Both double coincidence of wants and limited exchange describe features of barter.

- Correct description of barter.