Final answer:
A cold front signifies the boundary between colder and warmer air masses, leading to potential changes in weather, such as drops in temperature and the development of storms and precipitation.
Step-by-step explanation:
A cold front indicates the boundary between relatively colder and warmer air masses, so the correct answer to the student's question is 2) The boundary between relatively colder and warmer air masses.
Cold fronts are typically associated with low-pressure systems, where the atmospheric pressure at the earth's surface is lower than its surrounding environment. This leads to the rising and condensation of moist air, forming clouds and, often, precipitation. A cold front can cause a significant drop in temperature and bring various weather events like storms and sometimes even severe conditions such as tornadoes, depending on the difference in atmospheric pressures and other factors.
As an example, if a cold front blows into your locale, you could experience a dramatic drop in temperature, sometimes by as much as 40.0 Fahrenheit degrees. Weather maps used by meteorologists commonly display regions of high and low pressure, denoted by 'H' and 'L', and show cold fronts to aid in predicting such changes.