This rock turned out to be high grade uranium ore.

Question

Function is adjective

Uranium is a radioactive element that is used to create nuclear energy.

Function is Adjective

During the spring of 1950, a man bent down to pick up a yellow rock.

Function is Direct Object

New Mexico has petroleum, potash, copper, and natural gas-to name a few.
Function is Predicate Noun

Answer 1

None of these is right.

Please study the uses of adverbs.

Answer 2

During the spring of 1950, a man bent down to pick up a yellow rock.

Adverb <---

This rock turned out to be high grade uranium ore.

Predicate Noun <--- high grade uranium ore renames rock (is this right?)

Answer 3

Yes, that's an adverb.

I don't think turned out is a linking verb. I believe that to be high grade uranium ore is an adverb, modifying turned out. It tells how it turned out.