In the double entry system of book keeping a transaction affects:

The double entry system is designed to ensure that all financial transactions are recorded accurately and completely. It is also used to track the financial performance of a business.

Here is a brief explanation of each option:

• Option A: One account twice. This is not correct because every transaction affects two accounts.
• Option B: Two accounts at the same time. This is not correct because every transaction affects two accounts in two different ways.
• Option C: One account in two ways. This is not correct because every transaction affects two accounts in two different ways.
• Option D: Two accounts in two ways each. This is the correct answer because every transaction affects two accounts in two different ways.