I believe the linking verb is “will”. It’s linking the subject “the United States of America” and the predicate “be 300 years old!”
By definition, a business letter is used by employees and clients of different companies as a form of communication with regards to their agenda. In addition, it mainly comprises of tones having a much formal use of language. Proper formats must be observed in making these kind of letter such as avoid indenting when using a block format.
The answer to your question is A. - Them because you are referring to the crumbs and just not something random, in which case you would use "it".
I hope this helps you!
Have a great day!!
