The temperature at which the catalytic activity of the catalyst is maximum, is called:
(A) Critical temperature
(B) Room temperature
(C) Absolute temperature
(D) Optimum temperature

Hint: We know that the process in which the rate of the chemical reaction is altered by the use of different catalysts is known as catalysis. The chemical species which alter the rate of the reaction but are not consumed in the reaction are known as catalysts. The catalysts can be recovered from the chemical reaction. A small amount of catalyst is sufficient to carry out the reaction.

Complete step by step solution: We know that catalysts are an integral part of a chemical reaction. Catalysts alter the rate of a chemical reaction.
There are two types of catalysts: homogeneous catalysts and heterogeneous catalysts.
Homogeneous catalysts: The catalyst which has the same physical state as that of the reactant is known as a homogeneous catalyst.
Heterogeneous catalysts: The catalyst which has the same physical state different from that of the reactant is known as heterogeneous catalyst.
A positive catalyst decreases the activation energy of the reactant molecules. As a result the molecules react faster with each other and thus the rate of the chemical reaction increases.
The catalytic activity of the catalyst increases as the temperature increases. After a certain temperature, the catalytic activity of the catalyst starts decreasing.
At a particular temperature the catalytic activity of the catalyst is maximum. This temperature at which the catalytic activity of the catalyst is maximum is called optimum temperature.

Thus, the correct option is (D) optimum temperature.

Note: Example of homogeneous catalyst is reaction of sulphur dioxide gas with oxygen gas in presence of nitric oxide. The reaction leads to the production of sulphur trioxide gas.
${\text{2S}}{{\text{O}}_{\text{2}}}\left( {\text{g}} \right){\text{ + }}{{\text{O}}_{\text{2}}}\left( {\text{g}} \right){\text{ }}\xrightarrow{{{\text{NO}}\left( {\text{g}} \right)}}{\text{ 2S}}{{\text{O}}_{\text{3}}}\left( {\text{g}} \right)$
In the reaction, the catalyst, reactants and products all are in the same physical state .i.e. gaseous state.
Example of heterogeneous catalyst is reaction of sulphur dioxide gas with oxygen gas in presence of solid platinum metal. The reaction leads to the production of sulphur trioxide gas.
${\text{2S}}{{\text{O}}_{\text{2}}}\left( {\text{g}} \right){\text{ + }}{{\text{O}}_{\text{2}}}\left( {\text{g}} \right){\text{ }}\xrightarrow{{{\text{Pt}}\left( {\text{s}} \right)}}{\text{ 2S}}{{\text{O}}_{\text{3}}}\left( {\text{g}} \right)$
In the reaction, the catalyst is in solid state and the reactants and products are in gaseous state.