For plants and other organisms to develop differences among their respective species the alleles of their genes must contain variations. What is the original source of these variations?
A) Mutation
B) Crossing over
C) Random fertilization
D) Independent assortment

A) Mutation is the original source of variations for plants and other organisms  which develops differences among their respective species

Explanation:

Genetic variation leads to the natural differences between each individual and among individuals of a species. The main cause for this variation is due to alleles that cause variations in the composition of genes or the DNA sequences.  

A mutation is a genetic variation which leads to the creation of a new allele in a species population. Each variation in the DNA sequence leads to an allele formation and each allele leads to a difference in character or trait between individuals since they are the alternate version of the original genes.  

Although genetic variations could be brought about by crossing over, random fertilization, or independent assortment; mutation is the original source because it provides the basic raw material for all the evolutionary changes.

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products