Final answer:
Weather fronts separate air masses of different densities, not layers of the atmosphere or air masses of different altitudes. The correct answer is option b.
Step-by-step explanation:
Weather fronts separate air masses of different densities. These boundaries form between air masses that do not mix easily and have different temperatures and moisture levels.
The Earth's atmosphere is composed of several layers, including the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The troposphere is where most of the Earth's weather takes place, and it is the layer that is closest to the Earth's surface. Above it lies the stratosphere, and higher up are the mesosphere, thermosphere, and the outermost layer, the exosphere.
The correct answer to the question is option b: air masses of different densities. This is where the distinct properties of air masses can lead to the development of weather phenomena, such as storms and clear skies, depending on the nature of the interaction at the front.