Final answer:
The correct answer is option b. SO₄²⁻.
Step-by-step explanation:
An oxyanion is defined as a polyatomic ion that contains one or more oxygen atoms bonded to another element. Out of the given options, the only ion that is an oxyanion is SO₄²⁻ (sulfate ion).
The sulfate ion has four oxygen atoms bonded to a sulfur atom, giving it a negative charge of 2. It is commonly found in compounds such as sulfuric acid (H₂SO₄) and in the salts of sulfuric acid like sodium sulfate (Na₂SO₄).
In contrast, the other options do not meet the definition of an oxyanion. The fluoride ion (F⁻) is a monatomic anion consisting of a single fluorine atom. The ammonium ion (NH₄⁺) is a positively charged polyatomic ion consisting of one nitrogen atom and four hydrogen atoms bonded together. The thiocyanate ion (SCN⁻) is a polyatomic ion consisting of one sulfur atom, one carbon atom, and one nitrogen atom bonded together.