B) his sons, malcolm and donalbain
Alright, so amongst starters, you have a few you can use to show you'll have evidence:
According to text
According to the passage
According to the information provided
As the information shows
As the text mentioned, (include direct quote here)
Based on the information in the passage/text, (include reasoning here)
Etc. There are quite a few options, really, these are just a few. Based on whatever you are providing the information ON, the words information, text, and passage can be changed to suit your needs, such as article, newspaper, website, etc.
Formal writing is often used for academic and business work. It has a serious tone, facts, standard punctuation, and a third-person perspective. It does not include contractions, clichés or metaphors.