<u>Answer:</u> The correct answer is Option d.
<u>Explanation:</u>
Precipitation reaction is defined as the reaction in which an insoluble salt is formed when two solutions are mixed containing soluble substances. The insoluble salt settles down at the bottom of the reaction mixture.
For the given options:
- <u>Option a:</u>
![Rh^{3+}(aq.)+K_3PO_4(aq)](https://tex.z-dn.net/?f=Rh%5E%7B3%2B%7D%28aq.%29%2BK_3PO_4%28aq%29)
The chemical equation follows:
![Rh^{3+}(aq.)+K_3PO_4(aq.)\rightarrow RhPO_4(aq.)+3K^+(aq.)](https://tex.z-dn.net/?f=Rh%5E%7B3%2B%7D%28aq.%29%2BK_3PO_4%28aq.%29%5Crightarrow%20RhPO_4%28aq.%29%2B3K%5E%2B%28aq.%29)
As, no solid product is getting formed, it will not form any precipitate.
- <u>Option b:</u>
![Nb^{3+}(aq.)+Li_2CO_3(aq)](https://tex.z-dn.net/?f=Nb%5E%7B3%2B%7D%28aq.%29%2BLi_2CO_3%28aq%29)
The chemical equation follows:
![2Nb^{3+}(aq.)+3Li_2CO_3(aq.)\rightarrow Nb_2(CO_3)_3(aq.)+6Li^+(aq.)](https://tex.z-dn.net/?f=2Nb%5E%7B3%2B%7D%28aq.%29%2B3Li_2CO_3%28aq.%29%5Crightarrow%20Nb_2%28CO_3%29_3%28aq.%29%2B6Li%5E%2B%28aq.%29)
As, no solid product is getting formed, it will not form any precipitate.
- <u>Option c:</u>
![Co^{2+}(aq.)+Zn(NO_3)_2(aq)](https://tex.z-dn.net/?f=Co%5E%7B2%2B%7D%28aq.%29%2BZn%28NO_3%29_2%28aq%29)
The chemical equation follows:
![Co^{2+}(aq.)+Zn(NO_3)_2(aq.)\rightarrow Co(NO_3)_2(aq.)+Zn^{2+}(aq.)](https://tex.z-dn.net/?f=Co%5E%7B2%2B%7D%28aq.%29%2BZn%28NO_3%29_2%28aq.%29%5Crightarrow%20Co%28NO_3%29_2%28aq.%29%2BZn%5E%7B2%2B%7D%28aq.%29)
As, no solid product is getting formed, it will not form any precipitate.
- <u>Option d:</u>
![K^{+}(aq.)+Na_2S(aq)](https://tex.z-dn.net/?f=K%5E%7B%2B%7D%28aq.%29%2BNa_2S%28aq%29)
The chemical equation follows:
![2K^{+}(aq.)+Na_2S(aq.)\rightarrow K_2S(s)+2Na^+(aq.)](https://tex.z-dn.net/?f=2K%5E%7B%2B%7D%28aq.%29%2BNa_2S%28aq.%29%5Crightarrow%20K_2S%28s%29%2B2Na%5E%2B%28aq.%29)
As, potassium sulfide is getting formed as a solid. It is considered as a precipitate.
Hence, the correct answer is Option d.