An air pressure contour line joins locations in a region having the same atmospheric pressure. The following is an air pressure contour plot of a geographical region. Contour lines are shown at $0.05\text{ bar}$ intervals in this plot. If the possibility of a thunderstorm is given by how fast air pressure rises or drops over a region, which of the following regions is most likely to have a thunderstorm?

The rate at which air pressure rises or drops over a region is defined by the spatial pressure gradient:

1. In a contour map with a constant contour interval (here, $0.05\text{ bar}$), the distance between adjacent contour lines indicates the magnitude of the pressure gradient.

2. When contour lines are packed very closely together, the change in pressure per unit distance (gradient) is greatest.

3. Looking at the given regions:

   • Region R has the highest density of contour lines (lines are spaced closest to one another), which indicates the fastest spatial change in atmospheric pressure.

   • Regions P, Q, and S have wider spacing between contour lines, indicating a gentler pressure gradient.

Therefore, region R is most likely to experience a thunderstorm, which corresponds to option (C).