Which is not a unit of pressure:

The correct option is Kg/m².

To understand why this is the correct answer, let's look at the physical definition of pressure and analyze each of the given units.

Pressure ($P$) is defined as the force ($F$) applied perpendicular to the surface of an object per unit area ($A$) over which that force is distributed.

$$P=\frac{F}{A}$$

Let's evaluate each option based on this definition:
1. Bar: A bar is a widely recognized metric unit of pressure. One bar is defined as exactly 100,000 Pascals (Pa). It is frequently used in meteorology and industry.
2. N/m²: In the International System of Units (SI), force is measured in Newtons (N) and area is measured in square meters (m²). Therefore, Newtons per square meter (N/m²) is the direct unit of pressure (equivalent to 1 Pascal).
3. Torr: The torr is a non-SI unit of pressure defined as exactly $\frac{1}{760}$ of a standard atmosphere. It is historically and practically close to the pressure exerted by a 1 millimeter column of mercury (1 mmHg).
4. Kg/m²: Kilogram (kg) is a unit of mass, not force. Therefore, kilograms per square meter (Kg/m²) represents mass per unit area (also known as areal density or surface density), rather than force per unit area. For a unit to represent pressure using mass, it must incorporate gravitational acceleration (such as kgf/m², which is kilogram-force per square meter).

Since Kg/m² represents surface mass density and not force per unit area, it is not a unit of pressure.