Final answer:
For the best credit score, you should keep your debt-to-credit ratio below 30%. This is considered responsible use of credit and can lead to a higher credit score and better loan terms.
Step-by-step explanation:
You should aim to keep your debt-to-credit ratio below 30% for the best possible credit score. This is choice a) 30%. Maintaining a debt-to-credit ratio below this threshold is often advised by financial experts because it indicates to creditors that you're using credit responsibly and not overextending yourself. Furthermore, a lower credit utilization ratio can positively impact your credit score, making it easier to obtain loans at favorable interest rates in the future.