The correct answer is option 'D', which is Isotherm. Let's understand the concept of adsorption and its relation to pressure at constant temperature in detail.

Introduction to Adsorption:
Adsorption is the process of attracting and retaining molecules on the surface of a solid or liquid. The substance that gets adsorbed is called the adsorbate, and the surface on which adsorption takes place is called the adsorbent. Adsorption occurs due to weak intermolecular forces, such as van der Waals forces and hydrogen bonding.

Relation between Extent of Adsorption and Pressure:
The extent of adsorption is defined as the amount of gas adsorbed per unit mass of the adsorbent. It is denoted as x/m, where x is the amount of gas adsorbed and m is the mass of the adsorbent. The extent of adsorption depends on various factors, such as temperature, pressure, and the nature of the adsorbent and adsorbate.

At constant temperature, the relation between the extent of adsorption (x/m) and pressure is described by an isotherm. An isotherm is a graphical representation of the relationship between these two variables. It shows how the extent of adsorption changes with varying pressure while keeping the temperature constant.

Explanation of Other Options:
a) Isochore: Isochore refers to a graph that represents the relationship between volume and pressure at constant temperature. It is not related to adsorption.
b) Isomer: Isomer refers to compounds that have the same molecular formula but different structural arrangements. It is not related to adsorption.
c) Isobar: Isobar refers to a graph that represents the relationship between pressure and volume at constant temperature. It is not related to adsorption.

Importance of Isotherms:
Isotherms are crucial in understanding the behavior of adsorption processes. They provide valuable information about the adsorption capacity, the nature of adsorbent-adsorbate interactions, and the surface area of the adsorbent. Different types of isotherms, such as Langmuir, Freundlich, and BET isotherms, are used to characterize specific adsorption systems.

Conclusion:
In conclusion, the relation between the extent of adsorption (x/m) and pressure at constant temperature is called an isotherm. Isotherms are graphical representations that help us understand the adsorption process and determine important parameters related to it.