A salt may be prepared by the reaction of one salt with another salt, particularly if one of the products is insoluble.

In which one of the following reactions is this demonstrated?
(NH4)2CO3 + K2SO4 \(->NH4)2SO4 + K2CO3
CuCl2 + 2 NH4NO3 \->Cu(NO3)2 + 2 NH4Cl
Sr(NO3)2 + 2 KCl \->SrCl2 + 2 KNO3
BaCl2 + Na2S \->NaNO3 + KCl \->
NaCl + KNO3

I suggest two things here.

1. You need to re-read your post. The last two or three reactions appear to have been combined and as such they don't make sense.
#2. You need to know the solubility rules to know which salts are soluble. Here is a simplified set of solubility rules. I'll be glad to check your answer.
https://www.google.com/search?q=solubility+rules&client=firefox-a&hs=bg1&rls=org.mozilla:en-US:official&channel=sb&tbm=isch&imgil=eSgVJZM2Qry8hM%253A%253Bwuj7TohVYsLSMM%253Bhttp%25253A%25252F%25252Fyeahchemistry.com%25252Fsolubility-rules&source=iu&pf=m&fir=eSgVJZM2Qry8hM%253A%252Cwuj7TohVYsLSMM%252C_&usg=__Wv4bcTzS2j57NGbEUX3zndNDwy8%3D&biw=995&bih=601&ved=0CDEQyjc&ei=DNRSVK7_DYucNvzggegE#facrc=_&imgdii=_&imgrc=eSgVJZM2Qry8hM%253A%3Bwuj7TohVYsLSMM%3Bhttp%253A%252F%252Fyeahchemistry.com%252Fsites%252Fdefault%252Ffiles%252Fgeneralsolubilityrules.gif%3Bhttp%253A%252F%252Fyeahchemistry.com%252Fsolubility-rules%3B465%3B322

A salt may be prepared by the reaction of one salt with another salt, particularly if one of the products is insoluble.

In which one of the following reactions is this demonstrated?
(NH4)2CO3 + K2SO4 \->(NH4)2SO4 + K2CO3
CuCl2 + 2 NH4NO3 \-> Cu(NO3)2 + 2 NH4Cl
Sr(NO3)2 + 2 KCl \-> SrCl2 + 2 KNO3
BaCl2 + Na2S \-> BaS + 2 NaCl
NaNO3 + KCl \-> NaCl + KNO3

I meant to post it as this. thank you.
I believe the answer to be D (the last one?)

Here is a set of rules that may be a little easier to see.

http://www.csudh.edu/oliver/chemdata/solrules.htm

The revised post looks a lot better.
D is not the last one but it is the right answer. BaS is not soluble. All of the products from the other reactions are soluble. Check out the solubility page to confirm that. You need to practice these enough that the insoluble materials jump out at you.

Thank you Dr. Bob, I appreciate your help. I will look at the link. :D

In the given reactions, the reaction that demonstrates the preparation of a salt by the reaction of one salt with another salt, particularly if one of the products is insoluble, is:

CuCl2 + 2 NH4NO3 -> Cu(NO3)2 + 2 NH4Cl

In this reaction, copper chloride (CuCl2) reacts with ammonium nitrate (NH4NO3) to form copper nitrate (Cu(NO3)2) and ammonium chloride (NH4Cl). Copper nitrate is the desired salt formed from the reaction of two different salts, copper chloride and ammonium nitrate.

The product formed, ammonium chloride (NH4Cl), is an insoluble salt, indicating that this reaction meets the criteria mentioned.