Step 1: When sodium chloride reacts with conc. H₂SO₄, it forms hydrogen chloride (HCl) and sodium sulphate (Na₂SO₄).

The reaction that takes place is:

2NaCl + conc. H₂SO₄→ 2HCl + Na₂SO₄

Thus, A is concentrated H₂SO₄

Step 2: When hydrogen chloride reacts with iron metal (Fe), it forms iron (II) chloride (FeCl₂) and hydrogen gas (H₂).

The reaction that takes place is:

Fe + HCl → FeCl₂ + H₂

Thus, B is Fe.

Step 3: When hydrogen chloride reacts with ammonia (NH₃), it forms ammonium chloride (NH₄Cl).

The reaction that takes place is:

NH₃ + HCl → NH₄Cl

Thus, C is ammonia (NH₃).

Step 4: When hydrogen chloride reacts with lead nitrate [Pb(NO₃)₂], it forms lead chloride (PbCl₂) and nitric acid (HNO₃) .

The reaction that takes place is:

Pb(NO₃)₂ + HCl → PbCl₂ + HNO₃

Thus, D is lead nitrate [Pb(NO₃)₂]