The line on a weather map joining places of equal atmospheric pressure is termed ______.
A. Contour
B. Isohyet
C. Isotherm
D. Isobar

Hint: The term used to describe the line on a weather map joining the places of the same atmospheric pressure is also used in one of the branches of science, chemistry.

Complete answer: In meteorology, a branch of atmospheric science. The line on a weather map joining places of equal atmospheric pressure is termed the Isobar line. It is a line of equal or consistent pressure on a graph, map, or a plot. The magnitude and the direction of the wind field are linked to the distribution of the isobars.
While talking about the other options,
A. A line which joins points of equal height or elevation above a given level, for example, the mean sea level is known as the Contour line. Generally, it is just called Contour. The term Contour is used in cartography, the study and the practise of making maps.
B. On a map, the drawn line which connects points having equal rainfall at a certain time or a specific period of time is known as the Isohyet line.
C. A line is drawn on a map that joins points of the same temperature at a certain time or at a specific period of time is known as the Isotherm line.

Thus the correct option is D.

Note: Atmospheric chemistry and atmospheric physics are included in meteorology but the most important field for this branch of atmospheric sciences is weather forecasting. All these lines are used to predict the atmosphere for a given location and time.