which sample is a pure substance?\no zinc oxide\no sugar dissolved in water\no pond water\no soil

Brief Explanations:

A pure substance has a definite and constant composition. Zinc oxide is a compound with a fixed chemical formula (ZnO). Sugar dissolved in water is a solution (a mixture). Pond water contains various substances like dissolved gases, minerals, and microorganisms, so it's a mixture. Soil is a heterogeneous mixture of minerals, organic matter, etc.

Answer:

zinc oxide