Explanation:

- Option 1: PAN number

- The PAN (Permanent Account Number) is a unique identification code assigned by tax authorities in India to individuals and entities. It is used for tracking financial transactions and tax matters, not specifically for stock transactions.

- Option 2: Unique Order Code

- The Unique Order Code (UOC) is assigned to each transaction by the stock exchange. It helps track and identify transactions on a contract note.

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- Option 3: Contract Note

- A Contract Note is a legal document providing a record of a stock market transaction, but it is not a code.

- Option 4: None of the above

- Since the Unique Order Code is the correct answer, this option is not applicable.