[Steenrod]

An organic compound 'A' is treated with [itex]Sn/HCl[/itex] to get B. B is treated with (i)[itex]NaNO_2/HCl[/itex] and (ii) [itex]H_20][/itex] to phenol. Identify A and B.

I am unable to understand B can be treated with (i)[itex]NaNO_2/HCl[/itex] and (ii) [itex]H_20][/itex] to get phenol. My book mentions a reaction where [itex]NaNO_2/HCl[/itex] reacts with phenol to yield p-nitrosophenol in presence of nitric acid but I do not know how the current reaction can occur.

[discodermolide]

Looks like this is first a reduction of nitrobenzene to the aniline then diazotisation and quench to give phenol.
Look up the Sandmeyer reaction for the second part.

[orgopete]

An organic compound 'A' is treated with [itex]Sn/HCl[/itex] to get B. B is treated with (i)[itex]NaNO_2/HCl[/itex] and (ii) [itex]H_20][/itex] to phenol. Identify A and B.

I am unable to understand B can be treated with (i)[itex]NaNO_2/HCl[/itex] and (ii) [itex]H_20][/itex] to get phenol. My book mentions a reaction where [itex]NaNO_2/HCl[/itex] reacts with phenol to yield p-nitrosophenol in presence of nitric acid but I do not know how the current reaction can occur.

A can be nitrobenzene and B is aniline.

Aniline is diazotized to give a diazonium salt. If you draw out the structure, you should realize that a nitrogen is electron deficient, so it can accept electrons. Phenol is readily able to donate electrons so a new bond becomes formed between the C-4 of the phenol and the diazonium nitrogen to form a diazo compound. You should write out the mechanism to get all of the proton transfers correct.