Answer:
The number of STR repeats is unique and therefore it is considered as an allele of that <em>locus</em>
Explanation:
Short Tandem Repeats (STRs), also known as microsatellites or simple sequence repeats (SSRs), are short DNA sequences with a size of 1-6 nucleotide bases which may be many times repeated in tandem. STRs localize in specific regions of the genome (<em>loci</em>) and therefore they are molecular markers. Gel electrophoresis a technique used to separate DNA fragments based on their size. In consequence, the pattern of STR repeats or 'alleles' obtained by electrophoresis can be used to identify individuals. In a gel electrophoresis, STR markers produce different bands that run more slowly or faster on the gel in different lanes according to their size (e.g., more slowly >> higher size of the STR sequence), and thereby STR alleles are unique and serve to identify individuals.
I honestly think that a or d would be the best options. I would start with D and if that doesn't work try a.
Because their use is not sustainable because their formation takes billions of years to renew them
