The answer is B. Pressure.
Isobars are lines on a weather map that connect points of equal atmospheric pressure. They are used to show the distribution of pressure around the globe and to track the movement of weather systems.
Rainfall is the amount of water that falls to the ground as rain, snow, sleet, or hail. It is measured in inches or millimeters.
Population is the number of people living in a particular area. It is measured in people per square kilometer or square mile.
Height above sea level is the distance between a point on the Earth’s surface and the mean sea level. It is measured in meters or feet.
Isobars are not lines that join places having equal rainfall, population, or height above sea level.