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

A. I am in receipt of your order of February 10.
B. This letter is to make you cognizant of our receipt of your order.

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

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

Which do you think is the best sentence?

To determine the best opening sentence for a business letter that you want to sound fairly informal, let's analyze the options:

A. "I am in receipt of your order of February 10."
This sentence is more formal and uses a more technical language, which might not convey the desired informality.

B. "This letter is to make you cognizant of our receipt of your order."
Similar to option A, this sentence is also formal and uses complex language, making it less informal.

C. "Your order of February 10 arrived in our office today."
This sentence is straightforward and conveys the receipt of the order in a more casual manner, making it a potential choice for a fairly informal tone.

D. "Please be advised that I've just received your order."
This sentence is a bit more formal because of the phrase "please be advised," suggesting a more professional tone rather than informality.

Based on the given options, option C, "Your order of February 10 arrived in our office today," is the best opening sentence for a business letter that you want to sound fairly informal.