The antonym for "foolhardy" in the sentence is A:responsible.
Foolhardy means reckless or dangerous. Responsible means careful and sensible.
The other options are not antonyms for foolhardy:
Choosing is not an antonym for foolhardy. It means to select or decide on something.
Protect is not an antonym for foolhardy. It means to keep something safe or from harm.
Americans is not an antonym for foolhardy. It refers to the people of the United States of America.
Responsible means having a sense of duty to take care of things and make good decisions. It is the opposite of foolhardy, which means reckless and dangerous.
The sentence is saying that most Americans are being responsible by taking steps to protect the environment, while others continue to be reckless and dangerous by having wasteful habits.
The other answer choices are not antonyms for "foolhardy":
Choosing means making a decision.
Protect means to keep something safe.
Americans refers to the people of the United States.
Therefore, the correct answer is A.