Natural occurrence: the proportion of heavy water (D2O) naturally present in ordinary water is closest to which percentage?

Introduction / Context:
Ordinary water contains a small fraction of molecules in which deuterium replaces protium, forming heavy water (D2O) or semi-heavy water (HDO). Quantifying this natural abundance underpins heavy-water production processes and isotopic analyses in geoscience and biology.

Given Data / Assumptions:

• Natural deuterium abundance in the hydrosphere is roughly 0.015–0.016% by atoms.
• We express the order of magnitude as a simple percentage for MCQ purposes.
• Temperature/salinity variations cause only small local deviations.

Concept / Approach:
The tiny natural deuterium content means heavy-water production requires energy-intensive separation (e.g., Girdler sulfide or distillation). The figure most commonly cited in introductory texts is about 0.015%, which sets expectations for feedstock enrichment effort.

Step-by-Step Solution:
Recall canonical deuterium abundance ≈ 0.015–0.016%.Map this to the options; 0.015% matches closely.Choose 0.015.

Verification / Alternative check:
Isotopic ratio standards (VSMOW) indicate deuterium/hydrogen ratios consistent with ~0.015% abundance, validating the chosen value.

Why Other Options Are Wrong:

• 7.54 and 32.97: Orders of magnitude too high.
• 0.71: Associated with U-235 in natural uranium, not deuterium in water.
• 0.0015: Too low by about an order of magnitude.

Common Pitfalls:
Confusing U-235 abundance (0.71%) with deuterium; missing the percent versus fraction distinction.

Final Answer:
0.015