How Is a Reference Map Defined

How is a reference map defined?

A) An analytical tool to illustrate and emphasize the spatial variation of a particular theme or attribute
B) A map where dots or scale-adjusted symbols represent geographic phenomena
C) A map using colour or shading to indicated intensity of geographic phenomena
D) A map portraying the absolute locations of places and geographic phenomena using a standard frame of reference
E) A map using lines to connect locations of equal value with respect to a geographic phenomena