1. Identify the reaction type

The question refers to formation of a product $X$ from a nitrogen-containing aromatic compound. In this chapter, such questions commonly involve diazotisation of an aromatic amine followed by azo coupling, giving an azo dye/indicator.

A standard product formed in this way is methyl orange, obtained through:

• diazotisation of sulphanilic acid, then
• coupling with dimethylaniline.

2. Recognize the product $X$

The product formed is an azo compound containing the $-N=N-$ group. The well-known compound prepared this way and frequently asked in exams is methyl orange.

3. Use of methyl orange

Methyl orange is widely used:

• as an acid-base indicator in titrations.

So, if $X$ is methyl orange, then its use is:

$\boxed{\text{in acid-base titration as an indicator}}$

4. Check the options

• A: as food grade colourant
  Not correct for methyl orange.

• B: in acid base titration as an indicator
  Correct.

• C: in laboratory test for phenols
  Phenols are tested using reagents like neutral $\mathrm{FeCl_3}$, not methyl orange.

• D: in protein estimation as an alternative to ninhydrin
  Not correct.

5. Final answer

Thus, the product $X$ is used:

$\boxed{\text{B: in acid base titration as an indicator}}$