There are many types of pressure in everyday life. The measurements made to determine this type of pressure are usually related to ambient air pressure. There are some measurements made in relationships to vacuums. When you are trying to tell the difference between these zero references we find the terms, absolute pressure, gauge pressure and differential pressure very useful. Absolute pressure is referenced as zero versus a perfect vacuum. It is the gauge pressure plus the atmospheric pressure. The gauge pressure is also zero reference, but against ambient air pressure. The differential pressure is the difference in the pressure between two separate points.
Keeping all these facts in mind, the correct answer to your question is C Gauge Pressure. We would use D) N/m^2, to find the exact pressure. You take the measure of the pressure or force acting on the tire surface; (N) and the unit of measure on the surface is m^2. The term use to identify this type of pressure, if it is blood pressure, or tire pressure is Gauge pressure.
The answer is D) N/m^2, because we are talking about the pressure inside the tire, which has to be greater than the outside pressure, in order to keep up the shape of the tire.
The pressure is the result of a force which is acting on a surface, in this case, tire's surface. As we know, the unit of measure for force is Newton [N] and the unit of measure for surface is [m^2]. That's way, talking about the pressures, inside/outside, we are taking about Forces exercised on surfaces.
Explaining the process: If the pressures inside and outside the tire wouldn't be equal, then the shape of the tire would change, as one or the other of the pressures is greater. If the form is maintained constant, that means that the interior force which is pushing off orthogonally is equal with the exterior force which is pushing in, but forces having opposite directions.