Base-pair substitutions involving the replacement of a purine with a pyrimidine and vice versa are called:
a. isomers
b. transitions
c. transversions
d. inversions
e. translations

The answer is: c. tranversions

Explanation:

A mutation is the result of changes in the structure of a gen, these variations  happen in the nucleotide sequence of a genome, and can be as a consequence of DNA damage.

One type of DNA mutation is the substitution of one base pair for another. Two type of substitutions can happen. One of them are the transversions, which are interchanges of a purine (A or G) for a pyrimidine (C or T) and vice versa.

The other type of substitution are the transitions that are interchanges of one purine (A) for another purine (G) or a pyrimidine (C) to another pyrimidine (T).