The standard on which the atomic mass unit is based is the mass of a
a. proton.
b. neutron.
c. chlorine-35 atom.
d. carbon-12 atom.

Answer;

D. carbon-12 atom

Explanation;

-One atomic mass unit is the mass that is exactly equal to one-twelfth the mass of one carbon-12 atom. Therefore, the mass of carbon-12 is 12 amu and this provides the standard that is used in measuring the atomic mass of all other elements.

-The atomic mass of carbon is 12.01 and not 12.00 in the periodic table.

-Due to the fact that there are more than one element for most naturally occurring elements, the average mass of the naturally occurring mixture of isotopes is taken while determining the atomic mass of an element.

the answer is c.the valence of the atoms combine .