An isotope, x, of a particular element has an atomic number of 16 and a mass number of 34. therefore: select one:

a. x is an isotope of sulfur.

b. x has 18 neutrons per atom.

c. x has an atomic mass of 32.065.

d. a and

b.

e. a, b, and

c.

Number protons is what determines the element.
16 protons determines that the substance is sulphur.

The number of neutrons can vary according to the isotope.  The mass number is the total count of neutrons and protons, so sulphur with a mass number of 34 has 34-16=18 neutrons.

32.065 is the weighted average of the atomic weights of different isotopes, so does not apply to a single isotope.

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