Since this is a Main Conclusion question, we want to break down the stimulus in a way that clearly identifies the support relationships at play. Those support relationships are the most foolproof way to find the argument's conclusion: the conclusion will be supported by one or more premises, and the premise(s) will give us a reason to believe the conclusion.

In the stimulus, Helen discusses her brother Mark's excuse for missing their mother's birthday party. Helen makes three statements: (1) that it was wrong of Mark to claim that he missed the party because he was in a traffic accident; (2) that saying something false is always wrong; and (3) that Mark's excuse was false. Notice the types of statements: (1) is a value judgment about something Mark did, (2) is a principle that applies generally, and (3) is a factual claim about what happened to Mark.

These types of statements come together to form a rule-application argument pattern. In this structure, the premises are the rule and the factual statement, and the conclusion is the value judgment. That's because by applying a general rule to a specific situation, the speaker is able to support a judgment about that situation based on the rule.

For Helen's argument, that means that statement (1) is the conclusion, while statements (2) and (3) are support.