What do isobars on a surface weather chart represent?

Isobars on a surface weather chart represent lines of equal atmospheric pressure at the surface. This means that an isobar connects points of equal pressure, allowing meteorologists and pilots to understand pressure systems, identify high and low-pressure areas, and forecast weather conditions. The spacing between isobars can indicate the strength of the wind; closely spaced lines suggest strong winds, while widely spaced lines indicate weaker winds.

The other choices refer to different concepts. While lines reduced to sea level (option C, as suggested in the provided answer) do pertain to pressure, they are not the definition of what isobars directly represent on a surface weather chart. The correct concept of isobars excludes altitude considerations and temperature gradients, which are not indicated by isobars but rather by other atmospheric data on weather charts.