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 <em>how, that, what, whatever, when, where, whether, which, whichever, who, whoever, whom, whomever, </em>and<em> why, </em>which makes them easier to recognize.
The given sentence can be rewritten by introducing a noun clause related to the object (the United States): <em>What the author writes about thoughtfully are the United States.</em>