Answer:
a) The Killing of Banquo
Explanation:
The turning point for Macbeth happens when he orders the murder of Banquo. Banquo was a friend of Macbeth, but he knew about the witches' prophecy and he was told that his sons would be kings, this was a threat for Macbeth, so he decided to kill him and his son, Fleance. Banquo is murdered and Fleance managed to escape.
There are two actions happening at once in Act 3: One is the murder of Banquo and the other is the Banquet. The latter is connected with a very important dinner that the King Macbeth and the Queen are offering to the lords. Macbeth is truly tormented by the death of his "friend", so much so that he sees the ghost of Banquo sitting on his place and he makes a scene in front of everybody; he starts panicking, literally, like screaming and hiding behind a column. In fact, he starts talking about things that he shouldn't and Lady Macbeth is forced to cancel the banquet and send everyone home. This is why we say that this is his turning point, his madness gains another level and people are almost certain that he was the one that murdered the king and Banquo, because of his sketchy behaviour and the things he said.
Option B The murder of Duncan = Not possible because Macbeth was very stable regarding his mind, and he had almost everything under control.
Option C The murder of the Guards = Could be an option but it is not because although some people have suspicions, when  people find out that Macbeth killed them, no one can truly say that he did it to cover the murder of Duncan, although it is suspicious.  So, they still have it under control.
Option D The Murder of Macduff's family = This is important but it is not the turning point, specially because it happens almost at the end of the story, and turning points appear earlier on tragedies.
