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 ($ZnO$) is a compound with a definite chemical formula, so it's a pure substance. Sugar dissolved in water is a solution (mixture), pond water has various impurities (mixture), and soil is a mixture of minerals, organic matter, etc.

Answer:

A. zinc oxide