A cash discount received is entered on the three column cash book on:

The correct answer is A. Debit side.

A cash discount received is a reduction in the amount of money owed to a supplier when the invoice is paid within a certain time period. It is recorded as a debit in the cash book because it represents an increase in cash.

The other options are incorrect because:

• Option B is incorrect because a cash discount received is not a credit to the supplier.
• Option C is incorrect because a cash discount received is always recorded in the cash book.
• Option D is incorrect because a cash discount received is not written as a note.

I hope this helps! Let me know if you have any other questions.