Which observation could best be used to indicate an evolutionary relationship between two species?

They have similar base sequences.

Explanation:

The relationship between two species might be so close that the evolution of one species affects the evoluution of the other species.

Scientists compare the DNA base sequences of different species to determine evolutionary relationships. Species that share more similarities in their DNA bas sequences are more closely related than those who share fewere similarities.Scientists hypothesize that if two species have similarities in their base sequence,they share a common ancestor.

the similarities  in the organic bases sequence

Explanation:

Generally the DNA  of all organism contains sequences of  4-bases adenine, thiamine, Cytosine and Guanine.The information stored  in the DNA are usually encoded by the paining of theses bases.Therefore all organisms contain specific paring of these bases, and therefore have a unique DNA.

Encoded information from DNA  are transcribed as these bases to mRNA as Codons, and are translated when paired with complementary Anti codon on the tRNA.Therefore encoded information  are translated as Amino acids,with three bases  translating one Amino acid. Therefore the expression of theses bases is the major components of organism DNA  since it represented the genetic information needed to be expressed..

Therefore if an organism is investigated to  have evolved from another, definitely they must share  almost the same sequence of bases . Thus their DNA should  be closely match. This is what happens in DNA of chimpanzee,which is 98 % similar to man, because of similarities in the bases sequence in the two DNA s.