Final answer:
Gold (Au) is the element that occurs in nature only in compounds.
Step-by-step explanation:
The element that occurs in nature only in compounds is gold (Au).
Gold is a transition metal and it is found naturally in various compounds such as gold ore. It is highly unreactive and tends to exist in its elemental form as nuggets or in compounds with other elements.
For example, gold can occur in the compound gold sulfide (Au<sub>2</sub>S). So, option d. Au is the correct answer.