Statement a) Increase of pressure of a gas causes the amount of adsorption to increase:
When the pressure of a gas is increased, the gas molecules are forced closer together. This increases the chances of the gas molecules coming into contact with the surface of the adsorbent material and getting adsorbed. Therefore, an increase in pressure generally leads to an increase in the amount of adsorption. This is known as the Langmuir adsorption isotherm and is applicable for physical adsorption.

Statement b) Increase of temperature may increase or decrease the amount of adsorption:
The effect of temperature on adsorption depends on the type of adsorption. In physical adsorption, an increase in temperature generally decreases the amount of adsorption. This is because at higher temperatures, the kinetic energy of the gas molecules increases, causing them to have more energy to overcome the attractive forces of the adsorbent surface. As a result, the adsorption decreases.

On the other hand, in chemisorption, an increase in temperature may lead to an increase or decrease in adsorption depending on the specific reaction involved. Some chemisorption reactions are exothermic, meaning they release heat, and in such cases, an increase in temperature would decrease the adsorption. However, some chemisorption reactions are endothermic, meaning they absorb heat, and in these cases, an increase in temperature would increase the adsorption.

Statement c) The adsorption may be monolayer or multilayer:
Adsorption can occur in two main types of layers: monolayer and multilayer. In monolayer adsorption, a single layer of adsorbate molecules is formed on the surface of the adsorbent material. This layer is usually one molecule thick and is held onto the surface by weak van der Waals forces or physical interactions.

In multilayer adsorption, multiple layers of adsorbate molecules are formed on the surface of the adsorbent material. These layers can be stacked on top of each other and are held together by stronger chemical bonds or interactions. Multilayer adsorption can occur when the adsorbate molecules have a high affinity for the adsorbent surface or when the pressure of the gas is high enough to allow for the formation of multiple layers.

Statement d) Particle size of the adsorbent does not affect the amount of adsorption:
The particle size of the adsorbent can indeed affect the amount of adsorption. Smaller particle sizes generally have a larger surface area per unit mass compared to larger particle sizes. This increased surface area provides more sites for adsorption to occur, leading to a higher amount of adsorption.

Additionally, smaller particle sizes can also lead to a higher degree of accessibility for the adsorbate molecules, allowing them to come into closer proximity to the adsorbent surface and facilitating adsorption.

In summary, statements a, b, and c are correct, while statement d is incorrect. The increase in pressure generally leads to an increase in adsorption, the effect of temperature on adsorption depends on the type of adsorption, and adsorption can occur in both monolayer and multilayer forms. The particle size of the adsorbent does affect the amount of adsorption, with smaller particle sizes generally leading to higher adsorption.