Question

which sample is a pure substance?
○ zinc oxide
○ sugar dissolved in water
○ pond water
○ soil

Explanation:

A pure substance has a fixed composition and distinct properties. Zinc oxide ($\ce{ZnO}$) is a compound with a definite chemical formula, so it's a pure substance. Sugar dissolved in water is a mixture (solution), pond water has various impurities (mixture), and soil is a mixture of minerals, organic matter, etc.

Answer:

A. zinc oxide