Final answer:
The naturally occurring sulfur consists primarily of the isotope Sulfur-32.
Step-by-step explanation:
The naturally occurring sulfur consists primarily of the isotope Sulfur-32 (a), with a mass number of 32. This means that the majority of sulfur atoms have 16 protons and 16 neutrons. The other options, Sulfur-34 (b), Sulfur-36 (c), and Sulfur-38 (d), are not the dominant isotopes in naturally occurring sulfur.