What differs among dna structures of different species? a) the base pairs b) the nucleotide structure c) the types of amino acids used to make proteins d) the arrangement of the nucleotides within genes

It is the arrangement of nucleotide within the genes which differs among DNA structures of different species. Each gene contains several codons which is three nucleotide long. Several nucleotide are present in a DNA strand that is why we call it as polynucleotide. The sequence of nucleotide with in the DNA describes the sequence of mRNA and so is transcribed into proteins. Any change in the arrangement of nucleotide will change the final protein which is formed.    

clumped distribution, because you'de have all of the population gathered in a relatively small are.

it's the most common because it provides protection and a mate for reproduction

example: humans, seeweed, cattles, etc.