Lexical choice relates very closely to social distance and politeness strategies.
For example, a lack of social distance between two close friends may mean things like dysphemisms, diminutives, and reductions are highly used. However these specific lexical choices to say a letter to your employer for example would not fit the audience of the piece. Formality generally means a certain degree of social distance, and you'd like this social distance to be maintained by using certain speech strategies e.g. lexical choices that can upheld this level of formality. Providing specific examples on lexical choice in the passage they give you and explaining why they are used in this formal context as opposed to an informal context will probably get you marks.
So this should kind of be your formula:
Informal lexical choice-- > provide examples in text--> relate to social distance between two + participants --> effect of this lexical choice e.g. make friendship bond closer through using e.g. expletives, or intensifiers to express emotion, suggests friends are fairly close--> why a formal situation this informal lexis would be distance because of social distance
I hope this semi helps, you're on the right track.