Sodium phenoxide when heated with CO$_2$, under pressure at 125$^\circ$C yields a product which on acetylation produces C.
The major product C would be:

(a)

(b)

(c)

(d)

Hint: This question is based on the concept of acetylation reaction. As we can see in the question that B to C conversion takes place through the acetyl group. This is the Kolbe-Schmitt reaction, so now the product can be identified.

Complete step by step answer:
Now, we will move step by step for this reaction.
As we know, sodium phenoxide is the sodium salt of phenol.
The first step is when sodium phenoxide reacts with carbon-dioxide at high temp, and 5 atm to produce the salicylate i.e.


In the first step, we can see that phenol group is attached to the benzene ring, and COO- group is attached with the sodium ion.
Thus, the product formed in the first step is B.
Now, if we perform the further reaction to attain the product C, then there is a reaction of salicylate formed with the acid i.e. sulphuric acid in the presence of an acetyl group.
Thus, the product formed is


Here, we can see that –COOH group is attached at the ortho position, and the product formed is C i.e. salicylic acid.It is also known as aspirin.
The second step of this reaction is an example of acetylation reaction.
As mentioned the whole reaction i.e. from sodium phenoxide to the formation of salicylic acid is Kolbe-Schmitt reaction.
Hence, the major product formed C is


The correct option is (C).

Note: Don’t get confused while writing the complete chemical reaction, just perform it step by step as shown. There is a difference between salicylate, and salicylic acid. Salicylic acid is formed by the reaction of salicylate with sulphuric acid. So, salicylate acts as a starting material for acetylation reaction.