Paradise Lost refers to Adam and Eve, so the allusion stresses that story. 2. Charlie cannot read Paradise Lost anymore, which is a really difficult epic poem to read (as you heard in class).
'strict' and 'harsh' are out because business letters are not meant to insult or criticise, but rather to be courteous (polite). 'fancy' is out because business letters do not use unnecessarily flowery language.
as such, that only leaves us with 'formal' which is an appropriate tone for a business letter since it conveys a sense of seriousness.