Properties of Transition Elements

Steps:

• Identify nickel as a d-block element, confirming its transition metal status.
• Recall key properties: variable oxidation states, colored compounds, catalytic activity, and formation of complexes.
• Match these to the numbered options, assuming 1=variable oxidation states, 2=colored compounds, 3=catalytic activity, 4=forms complexes (based on standard lists).
• Select combination including 1, 2, and 3 as correct for nickel.

Why A is correct:

• Nickel exhibits variable oxidation states (e.g., +2, +3), forms colored compounds (e.g., NiSO4·6H2O is green), and acts as a catalyst (e.g., in hydrogenation), aligning with transition metal definitions.

Why the others are wrong:

• B includes 4 but omits 2; nickel's colored compounds are a core property.
• C excludes 3; nickel's catalytic role is essential.
• D excludes 2; color in compounds is definitive.

Not enough information: Properties 1–4 not specified in query.

Final answer: A