Explanation:
Passivity is a phenomenon in which the metal surface becomes unreactive towards the environment due to the formation of a thin layer of oxide, which acts as a protective layer. The passivity of a metal depends on its ability to form a stable oxide layer in the presence of an oxidizing agent like HNO3.

Metals that show passivity with cone HNO3:

Out of the given metals, the following 4 metals show passivity with cone HNO3:

1. Chromium (Cr): Chromium is known for its excellent corrosion resistance due to the formation of a thin and stable layer of chromium oxide on its surface. This layer protects the metal from further corrosion and makes it passive towards HNO3.

2. Nickel (Ni): Nickel also forms a passive oxide layer on its surface, which protects it from corrosion. This layer is composed of nickel oxide, which is stable and insoluble in HNO3.

3. Copper (Cu): Copper is known for its excellent resistance to HNO3 due to the formation of a protective layer of copper nitrate on its surface. This layer prevents further reaction of copper with HNO3 and makes it passive.

4. Silver (Ag): Silver also forms a passive oxide layer on its surface, which protects it from corrosion. This layer is composed of silver oxide, which is stable and insoluble in HNO3.

Metals that do not show passivity with cone HNO3:

The following 4 metals do not show passivity with cone HNO3 and are easily corroded by it:

1. Iron (Fe): Iron reacts with HNO3 and forms iron(III) nitrate, which dissolves in HNO3, leading to further corrosion.

2. Zinc (Zn): Zinc reacts with HNO3 and forms zinc nitrate, which dissolves in HNO3, leading to further corrosion.

3. Aluminum (Al): Aluminum reacts with HNO3 and forms aluminum nitrate, which dissolves in HNO3, leading to further corrosion.

4. Tin (Sn): Tin reacts with HNO3 and forms tin(IV) nitrate, which dissolves in HNO3, leading to further corrosion.

Therefore, the correct answer is '4' as only 4 out of the given 8 metals show passivity with cone HNO3.