This question involves identifying intermediate compounds B and C in a multi-step organic conversion. The reaction sequence appears to be a classic transformation of aniline (A) through diazotization and subsequent reactions.

Step 1: Formation of B from Aniline (A)
Aniline (C₆H₅NH₂) undergoes diazotization when treated with NaNO₂ and HCl at low temperature (0-5°C). This replaces the -NH₂ group with -N₂⁺Cl^-, forming benzene diazonium chloride (C₆H₅N₂Cl). Thus, compound B is benzene diazonium chloride.

Step 2: Formation of C from B
Benzene diazonium chloride (B) reacts with hypophosphorous acid (H₃PO₂). This is a reduction reaction where the diazonium group (-N₂⁺) is replaced by a hydrogen atom (-H). The product is benzene (C₆H₆). Thus, compound C is benzene.

The overall conversion is:
Aniline (A) $\overset{NaNO 2 / HCl, 0 - 5 ° C}{\to}$ Benzene diazonium chloride (B) $\overset{H 3 PO 2}{\to}$ Benzene (C)

Therefore, B is benzene diazonium chloride and C is benzene.

Related Topics:
This conversion involves concepts from diazotization reactions and reduction of diazonium salts. Diazotization is a key reaction for converting aromatic amines to various other functional groups. The reduction with H₃PO₂ is a specific method to replace the diazonium group with hydrogen.

Important Formulae/Reactions:
Diazotization: Ar-NH₂ + NaNO₂ + 2HCl → Ar-N₂⁺Cl^- + NaCl + 2H₂O (at 0-5°C)
Reduction with H₃PO₂: Ar-N₂⁺Cl^- + H₃PO₂ + H₂O → Ar-H + N₂ + H₃PO₃ + HCl

Looking at the provided options, the correct pair is:
B = C₆H₅N₂Cl (benzene diazonium chloride)
C = C₆H₆ (benzene)