Based on this phylogenetic tree, which species is most likely closely related to bos taurus?

The correct answer is Ovis aries.

Explanation:

In a phylogenic tree the closely related species are identified on the basis of most number of common ancestors. The species sharing the maximum number of the common ancestors are said to be closely related. They exhibit more genetic similarity as well as physical traits. In the given phylogenetic tree, Ovis aries and Bos taurus share a same ancestors as compared to others. Hence, they are most closely related.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.