Urey and Miller's experimental setup was able to produce all twenty of the amino acids found in proteins. Some amino acids contain the element oxygen. Which starting material in the Urey-Miller experiment was the source of this element?

The source of the oxygen in amino acids was water (H2O).

Explanation:

The Miller-Urey experiment was a chemical experiment in which the hypothetical conditions present on the early Earth were simulated. This experiment was conducted in order to test what kind of environment would be needed to allow life to begin. The experiment is considered to be the classic experiment on the origin of life. It was conducted in 1953 by two scientists; Stanley L. Miller and Harold C. Urey.

In this experiment Miller and Urey placed water (H2O), methane (CH4), ammonia (NH3) and hydrogen (H2) into an enclosed chamber. The chamber was set up to mimic the atmosphere of the early Earth, with a reducing environment and high amounts of energy (lightning storms and high temperatures). At the end of the experiment Miller and Urey observed the spontaneous synthesis of amino acids, life-essential compounds, and concluded that organic molecules needed for life could be formed from inorganic components.

Starting materials used in this experiment served as a source of different elements needed for synthesis of amino acids. The source of the element oxygen in amino acids was water (H2O) that was put inside the chamber.