Match List I with List II

	List-I (Ion)		List-II (Group Number in Cation Analysis)
A.	$\text{Co²⁺}$	I.	Group-I
B.	$\text{Mg²⁺}$	II.	Group-III
C.	$\text{Pb²⁺}$	III.	Group-IV
D.	$\text{Al³⁺}$	IV.	Group-VI

The correct option is A-III, B-IV, C-I, D-II.

In qualitative inorganic analysis, cations are classified into groups based on their precipitation behavior with specific group reagents. Let us analyze each cation from List-I and identify its corresponding group in List-II:

1. Cobalt ion:
${\text{Co}}^{2+}$
Cobalt belongs to Group-IV of basic radicals. The group reagent for Group-IV is hydrogen sulfide gas:
${\text{H}}_2\text{S}$
in the presence of ammonium chloride:
${\text{NH}}_4\text{Cl}$
and ammonium hydroxide:
${\text{NH}}_4\text{OH}$
Under these alkaline conditions, cobalt precipitates as cobalt sulfide:
$\text{CoS}$
Therefore, A matches with III.

2. Magnesium ion:
${\text{Mg}}^{2+}$
Magnesium belongs to Group-VI of basic radicals. Since it does not precipitate with the group reagents of Group-I to Group-V, it is tested in Group-VI using disodium hydrogen phosphate:
${\text{Na}}_2{\text{HPO}}_4$
in the presence of ammonium hydroxide:
${\text{NH}}_4\text{OH}$
This forms a white crystalline precipitate of magnesium ammonium phosphate:
${\text{MgNH}}_4{\text{PO}}_4$
Therefore, B matches with IV.

3. Lead ion:
${\text{Pb}}^{2+}$
Lead belongs to Group-I of basic radicals. The group reagent for Group-I is dilute hydrochloric acid:
$\text{HCl}$
Lead is precipitated as a white precipitate of lead chloride:
${\text{PbCl}}_2$
Therefore, C matches with I.

4. Aluminum ion:
${\text{Al}}^{3+}$
Aluminum belongs to Group-III of basic radicals. The group reagent is ammonium hydroxide:
${\text{NH}}_4\text{OH}$
in the presence of ammonium chloride:
${\text{NH}}_4\text{Cl}$
Aluminum precipitates as a gelatinous white precipitate of aluminum hydroxide:
$\text{Al}{\left(\text{OH}\right)}_3$
Therefore, D matches with II.

Matching Summary:
• A. ${\text{Co}}^{2+}$ → III. Group-IV
• B. ${\text{Mg}}^{2+}$ → IV. Group-VI
• C. ${\text{Pb}}^{2+}$ → I. Group-I
• D. ${\text{Al}}^{3+}$ → II. Group-III