What is the best opening sentence for a business letter that you want to sound informal?

The purpose of the opening (first sentence) of any business letter is to tell the recipient why you are writing.

You can be formal:

I'm writing to inquire...

I'm writing to follow up our conversation about...

I'm writing to advise you...

I'm writing to request...

Or, you can be informal:

I'd like to ask... (or find out...)

I'd like to follow up with you on...

I'm letting you know...

I'd like to request...

But never be too informal, even if you are writing to a personal friend or family member on a business subject; or someone you have done business with for some time. The recipient may have to involve another person to follow up your letter, or your letter may become important in another setting in the future. The content must be clear and concise even when it is informally written.

The full question is:

Which of the following is the best opening sentence for a business letter that you want to sound fairly informal?

a. Please be advised that I've just received your order.

b. Your order of February 10 arrived in our office today.

c. This letter is to make you cognizant of our receipt of your order.

d. I am in receipt of your order of February 10.

The best choice for an informal tone would be:

b. Your order of February 10th arrived in our office today.