A cline: a. is continuous variation that follows a geographic continuum.
b. is variation that falls into discrete categories.
c. was used to describe human variation as early as the Renaissance.
d. describes the pattern of occurrence of trisomy 18.

The correct answer is - option A is continuous variation that follows a geographic continuum.

Explanation:

A cline is a slow change in the physical and genetic traits of a population or species across a geographical difference or gradient. If a species present in a very large area, there would be some members live on different environmental condition in comparison to others.

By this gradient members of species show continuous variation that follows a geographic continuum. For, example, In a species of warm-blooded animals have large body size in cooler area than in warmer area.

Thus, the correct answer is - option A is continuous variation that follows a geographic continuum.

phaneritic textures in which phaner means visible so phaneritic textures are standard of impertinent volcanic rocks. these rocks crystallized gradually under the surface of earth. when the magma cools deliberately then the ores have chance to develop and produce long crystals.