Answer:
What the author writes about thoughtfully are the United States.
Explanation:
Noun clauses are dependent (also called subordinate) clauses that act as nouns. Accordingly, they can function as the subjects, direct and indirect objects, predicate nominatives, and objects of a preposition of sentences, just like nouns. They begin with words such as how, that, what, whatever, when, where, whether, which, whichever, who, whoever, whom, whomever, and why, which makes them easier to recognize.
The given sentence can be rewritten by introducing a noun clause related to the object (the United States): What the author writes about thoughtfully are the United States.
