The president invited Senator Smith to the White House for a meeting before preparing his address to the United States congress.

Which of the following is an error in capitalization in the sentence above?
The word “president” should be capitalized
The word “Senator” should be lower case
The word “House” should be lower case
The word “congress” should be capitalized

I was thinking D

I agree.

By the way, language rules do change over time. We used to have to capitalize president when it referred to the U.S. president. That "rule" has fallen out of favor with most grammarians.

Yes, you are correct. The error in capitalization in the sentence is option D: The word "congress" should be capitalized.

To find the error, you need to understand the general rules of capitalization. In this sentence, "president" should be lowercase because it is not being used as a title or as part of a proper noun. "Senator" should be capitalized because it is used as a title before a specific name, which is Senator Smith. "House" should be capitalized because it is part of the proper noun "White House." Finally, "congress" should be capitalized because it refers to the specific governing body, which is the United States Congress.

So, by process of elimination, the correct answer is option D: "The word 'congress' should be capitalized."