Yes you are correct! It is c
Answer:
The Americans
Explanation / Resources:
"The United States received the disputed Texan territory, as well as New Mexico territory and California. The Mexican government was paid $15 million — the same sum issued to France for the Louisiana Territory. The United States Army won a grand victory." (www.ushistory.org)
"The Mexican-American War was a conflict between the United States and Mexico, fought from April 1846 to February 1848. Won by the Americans" (www.britannica.com)
It's a nonessential word, clause, or sentence added to an already grammatically complete sentence. It's most commonly set off by brackets, but it can also be set off by commas or dashes.
Example:
My mother went to the supermarket and picked up some strawberries (my favorite fruit) earlier today.