Which geologic principle explains why the bottom stratum in a section of rock is the oldest? . cross-cutting relationships . superposition . unconformities . uniformitarianism .

Superposition is the geologic principle which explains why the bottom stratum in a section of rock is the oldest. The word literally means placing one thing on top of another where the lower strata are the older. 

Unconformities

Explanation:

An unconformity is a geologic principle that was given by James Hutton in 1788, which states that surfaces separating two stratum or rock masses of different ages indicate that sediment deposition was not continuous, in general, the older layer were subjected to erosion and then depositions of the younger layer.  Thus it describes a break in the geologic structure from the older to the younger hence its a kind of relative dating of rocks.

Ithink the answer to that would be b):