(1) Over the past 20 years, Hormone Replacement Therapy has been used to give new life to ageing people, primarily by raising their energy levels. (2) However, research indicates that the hormone dosage used in the past had extremely negative health effects. (3) Doctors want to stop using this type of therapy altogether.

The argument's conclusion appears in which sentence(s)?

In sentence 1?
Incorrect. One way to find the conclusion is by finding the premise. What is not the premise is the conclusion and vice versa. The first sentence, Over the past 20 years... presents us with facts and facts are usually the premise. Therefore, sentence 1 cannot be the conclusion.

In sentence 2?
Not quite. One way to find the conclusion is by finding the premise. What is not the premise is the conclusion and vice versa. The second sentence, Research indicates... presents us with facts and facts are usually the premise. Therefore, sentence 2 cannot be the conclusion.

Sentence 3?
[spoiler]Good![/spoiler] Another way to find the conclusion is by asking,"what do these facts (the premise) lead to?" Try putting in the conjunction therefore in front of the sentences. Where would therefore most fit? The only place in which therefore fits is in front of sentence 3, as the result of hormone replacement therapy (sentence 1) and the research (sentence 2) lead to what the doctors decided. Decisions, actions, and beliefs on the basis of facts usually indicate the conclusion.