Which of the following elements is classified as a trace element that cells need in small quantities?

The classification of elements as trace elements is based on the quantities in which they are required by cells. Trace elements are essential for various biological processes but are needed in very small amounts. In this context, oxygen is not classified as a trace element because cells require it in much larger quantities for critical processes such as cellular respiration. Oxygen is a major component of the atmosphere and is necessary for aerobic organisms to produce energy.

On the other hand, both iron and copper are indeed examples of trace elements because they are needed in small amounts for functions such as enzyme activity and electron transport. Calcium, while critical for various cellular functions such as signaling, muscle contraction, and maintaining structural integrity in organisms, is typically required in larger quantities than trace elements. Thus, the correct option reflects an understanding of the specific requirement levels for these elements in cellular biology.