Which of the words in the following sentence signals the use of informal language? I would appreciate it if more information could be sent.

A. Appreciate

B. Could

C. I

D. Sent

C is the correct answer.

In formal writing, you typically wouldn’t write in first person (unless you are specifically instructed to.) "I" would make a text sound like it has no credibility, because it’s basically stating an opinion (ex. "I believe ... " "I think that ... ")

I, because I is not used in formal writing.