Identify the extra element being estimated in Carius method

In Carius estimation, the organic compound is heated in a sealed Carius tube with a strong oxidizing agent.

Here, the compound is treated with sodium peroxide to form a mineral acid $Y$. Then $\mathrm{BaCl_2}$ is added and a precipitate $Z$ is formed.

The precipitate formed with $\mathrm{BaCl_2}$ is typically: $\mathrm{BaSO_4}$ which is used in the quantitative estimation of sulphur.

So: $Y = \mathrm{H_2SO_4}, \qquad Z = \mathrm{BaSO_4}$

Therefore, the extra element in $X$ must be sulphur.

Check each option for presence of sulphur

A: Cytosine
Cytosine is a nitrogenous base containing C, H, N, O but no sulphur.

B: Methionine
Methionine is an amino acid with structure containing: $-S-$ Hence it contains sulphur.

C: Chloroxylenol
Chloroxylenol contains chlorine, oxygen, carbon, hydrogen, but no sulphur.

D: A nucleotide
A nucleotide generally contains C, H, O, N, P, but not necessarily sulphur. In the usual sense, it is not taken as a sulphur-containing compound.

Conclusion

The compound $X$ must contain sulphur, so the correct option is: $\boxed{\text{B: Methionine}}$

Comparison with stored answer

Stored correct answer: B
Derived answer: B

Hence, the derived answer agrees with the stored answer.