The correct answer is option 4, Isohyets. Isohyets are lines that join places of equal rainfall. They are used in meteorology and hydrology to map the distribution of rainfall in a particular area. These lines are similar to contour lines on a topographic map, except that they represent rainfall instead of elevation. Isohyets help scientists and researchers analyze patterns of rainfall and understand how it varies across different regions. By studying these lines, they can identify areas with high or low rainfall and make predictions about future weather patterns. It is important to note that the other options mentioned in the question are not related to rainfall. Isohypses represent lines of equal elevation, isohalines represent lines of equal salinity, and isobars represent lines of equal atmospheric pressure.