Macbeth believeS that malcolm cannot harm him because the spirits predicted that no man born of a woman could harm macbeth. Option A is correct.
The witches explained to Macbeth that he will not be murdered by anyone born of a woman. Macbeth interpretation of that explanation is that he cannot be killed by anyone, due to the fact that everyone is delivered and born of woman. So that, Macbeth is not intimidated by Malcolm.