Answer:
b. He knows Macduff has fled to England.
The reason why Macbeth initially change his mind and decided not to have Macduff killed after visiting the witches after Act IV is because he knows Macduff has fled to England. After Macbeth left the witches' house, Macbeth decides that Macduff and his family need to be killed because they are a threat to him and will attempt to overthrow him. So, Macbeth sends killers to kill the Macduff family and take his kingdom.