Frozen dew forms when: a. water vapor changes phase directly to ice
b. frost forms, then the dew point falls below freezing
c. liquid dew forms, then the temperature falls below freezing
d. water freezes directly to hygroscopic condensation nuclei in the lower atmosphere

The answer is
(B) Frost forms, then the dew point falls below freezing.

... the relationship between a dependent variable and one or more independent variables. A model of the relationship is hypothesized, and estimates of the parameter ... The data are shown graphically in the figure below, called a scatter diagram. ... analyses can be interpreted as establishing cause-and-effect relationships.