What is the difference between a. Two clients from Peru will be visiting us next month when Margaret returns. and b. Two clients from Peru will visit us next month when Margaret returns. and c. Two clients from Peru are going to visit us next month when Margaret returns. and d. Two clients from Peru are visiting us next month when Margaret returns. ?Feel free to just provide example sentences.

@tatsuca yes A is the correct one B is not bad either, like if you were to say this it will not sound unnatural it just not well written if it's for a formal context ( maybe someone else can provide a better answer)

C and D is incorrect because 'is going to visit us' is talking about one person a singular subject ( if the sentence was about one client visiting then C is fine)
Correction ' are going to visit us'

same with D correction is
' are visiting us'
instead of 'is visiting us'
- since you are speaking about 2 people