**Stoichiometric Quantity of Air**

The stoichiometric quantity of air refers to the amount of air required for complete combustion of a fuel with no excess air. It is the theoretical amount of air needed to completely react with all the fuel molecules present, resulting in the complete conversion of fuel to carbon dioxide and water.

**Explanation**

**a) Air present in the atmosphere at NTP conditions**
This option is incorrect because the air present in the atmosphere at normal temperature and pressure (NTP) conditions is not the stoichiometric quantity of air. The air in the atmosphere consists of various gases such as nitrogen, oxygen, carbon dioxide, and traces of other gases, but it does not have a specific stoichiometric ratio.

**b) Air required for complete combustion of fuel with no excess air**
This option is the correct answer. The stoichiometric quantity of air is the exact amount of air required to completely burn a fuel without any excess air. It is based on the chemical reaction between the fuel and oxygen in the air, following the stoichiometric ratio of the reaction equation. This ensures that all the fuel is converted into carbon dioxide and water without any unburned fuel or incomplete combustion products.

**c) Air required for optimum combustion so as to have reasonable excess air**
This option is incorrect because it refers to the air required for optimum combustion, which includes a reasonable amount of excess air. The stoichiometric quantity of air does not include any excess air, as it is the minimum amount required for complete combustion.

**d) Air required to convert CO into CO2**
This option is incorrect because it refers to the air required for the conversion of carbon monoxide (CO) to carbon dioxide (CO2). While air is indeed required for this reaction, it is not the stoichiometric quantity of air, as the stoichiometric quantity is specifically for the complete combustion of the fuel.

**e) Air required to form an explosive mixture**
This option is incorrect because an explosive mixture requires a fuel, oxygen, and an ignition source. The stoichiometric quantity of air is not necessarily related to explosive mixtures, as it is focused on the complete combustion of the fuel rather than the conditions for explosive reactions.

In conclusion, the correct answer is option 'b' - the stoichiometric quantity of air refers to the air required for complete combustion of fuel with no excess air. It is the theoretical amount of air needed to fully react with the fuel and convert it into carbon dioxide and water.