Before precipitating group III, the filtrate from group II is boiled with a few drops of conc. $HNO X_{3}$ . The main purpose of the nitric acid is

Ato oxidise $Fe X^{2 +}$ to $Fe X^{3 +}$ so that iron is completely precipitated as $Fe (OH) X_{3}$
Bto convert $Al X^{3 +}$ to $Al (NO X_{3}) X_{3}$ which precipitates
Cto precipitate $Fe X^{2 +}$ as $Fe (NO X_{3}) X_{2}$
Dto reduce $Cr X^{3 +}$ to $Cr X^{2 +}$

Answer: A

Iron may be present as

Fe X^{2 +}

(the

H X_{2} S

reduces

Fe X^{3 +}

Fe (OH) X_{2}

has a much larger

K_{s p}

and would not precipitate completely at the low

[OH X^{-}]

of the buffer. Oxidation to

Fe X^{3 +}

makes the very insoluble

Fe (OH) X_{3}

form fully.