Soda extract is useful when given mixture has any insoluble salt , it is prepared by :
A.Fusing soda and mixture and then extracting with water
B.Dissolving $NaHC{O_3}$ and mixture in dil $HCl$
C.Boiling $N{a_2}C{O_3}$ and mixture in dil $HCl$
D.Boiling $N{a_2}C{O_3}$ and mixture in distilled water

Hint: Soda extract is alkaline in nature because the organic compound is fused with sodium metal and then it is extracted with water . Hence , the unreacted metal reacts with water and forms an alkaline solution .

Complete step by step answer:
Soda extract or sodium carbonate extract is prepared when any insoluble salt is present that is , it is partially or completely insoluble in water.
It is prepared by mixing the salt with pure $N{a_2}C{O_3}$ in the ratio of $1:2$ and sufficient amount of distilled water is added to it.
Then the whole content is boiled and filtered. This filtrate is used as soda extract.
$C{a_3}{(P{O_4})_2} + 3N{a_2}C{O_3} \to 3CaC{O_3} + 2N{a_3}P{O_4}$

So, the correct answer is Option D.

Additional Information:
Some advantages of preparing this extract are:
1)The cations which interfere in the confirmatory tests of some anions are removed in the form of insoluble carbonates as a result of treatment with sodium carbonate.
2)Those anions can be identified easily in such mixtures which are insoluble in water and dilute

Note:
The main reason to prepare soda extract is that the elements in an organic compound are found in covalent form , in which they cannot be detected .Therefore with the help of soda extract we convert them into ionic form so that they can be detected .