Happy to help. To find the conclusion, always think to yourself of what sentence or phrase is being supported by the rest of the stimulus. Additionally, if you're stuck and can't figure out the conclusion between two sentences, a good trick is to put the word "because" in between the sentences and then see which way makes more sense.
We're told that it was wrong for Mark to tell their brother that he missed the party because of an accident. Helen says that saying something false is always morally wrong, and the reality was that there wasn't an accident (so Mark said something that was false).
Helen is using a general principle (that it's always wrong to say something that's false) to support her conclusion that it was wrong for Mark to tell his mother a false reason for missing the party.
We're looking for the answer choice that gives us the main point, which in this stimulus is that it was wrong for Mark to give their mother a false reason for missing the party.
(A) says, "Mark did not tell his mother the truth"
The problem with (A) is that simply saying that Mark did not tell the truth is not the same thing as saying that Mark was wrong. (A) is a premise supporting the notion that Mark was wrong for his action. Thus, (A) is out.
(D) says, "it was wrong of Mark to tell his mother that he had missed her birthday party as a result of having been in a traffic accident"
(D) is great. It provides us with a statement that claims that Mark was wrong for lying to his mother, and this is precisely what we identified the conclusion of the stimulus as being, so this is the correct answer choice.
Does this make sense? Let us know if you have any other questions!