The question as provided is incomplete/corrupted. It states:

• Three isomers $A, B, C$ of molecular formula $C_8H_{11}N$
• They give “the following results: R has lower boiling point than S”
• Then asks “$A, B$ and $C$, respectively are :”
• But the actual identities/options for $A,B,C$ are missing.

So, a fully rigorous determination from the visible text alone is not possible.

However, let us analyze what can still be inferred.

1. Degree of unsaturation

For $C_8H_{11}N$:

Thus, the molecule has $4$ degrees of unsaturation, which strongly suggests a benzene ring.

So these are most likely aromatic amine-type isomers such as:

• ethylaniline isomers,
• xylidine isomers,
• benzylmethylamine / phenethylamine type isomers,
• toluidine-type related constitutional isomers.

2. Boiling point clue

The visible clue says: “R has lower boiling point than S”.

In isomeric amines, boiling point depends largely on:

1. intermolecular hydrogen bonding, and
2. extent of branching / shape.

Typical trend:

• Primary amines usually have higher boiling points than secondary/tertiary isomers of comparable molar mass because they form stronger intermolecular H-bonding.
• More branched amines usually have lower boiling points than less branched ones.
• If one isomer can form intramolecular association or is sterically hindered, its boiling point may be lower.

So if the original question included some reaction sequence identifying $R$ and $S$, then the lower boiling point of $R$ could be used to distinguish among primary/secondary amine isomers.

3. Why the exact answer cannot be derived from the visible data

To determine $A,B,C$ uniquely, we would need at least one of the following, which are absent here:

• structures/options,
• reaction conditions,
• identities of $R$ and $S$,
• any nitration/diazotization/Hinsberg/carbamine test data,
• melting/boiling point comparisons among the actual isomers.

Since none of this is visible, there is not enough information to independently reconstruct the answer.

4. Comparison with stored answer

The stored correct answer is D.

Because the options themselves are missing, I cannot verify independently that option $D$ is correct from the visible statement alone.

So I must conclude that the question text is insufficient for a proper derivation.

5. Final conclusion

From the provided data alone, the answer cannot be uniquely determined. The question appears truncated.

Therefore, I do not have sufficient basis to agree confidently with the stored answer $D$, even though it may well be correct in the original complete version of the problem.