The atomic mass of chlorine is 35.45 u. is it possible for any single atom of chlorine to have a mass number of exactly 35.45? explain mathematics

No, this number is the result of the natural mixture of Cl-35 and Cl-37, which produces the observed value.

The mass of Cl on the periodic table is 35.45 atomic mass units. This is an average mass of the isotopes (different Cl atoms in nature). Most Cl atoms in nature have a relative mass of about 35 atomic mass units. Most of the remaining Cl isotopes have a relative mass of 37 atomic mass units. 

The answer is the first option just took the test