Which of the following trace elements is needed in microgram quantities?

Manganese is classified as a trace element, which means it is required by organisms in very small amounts, typically in microgram quantities. It plays a vital role in several biological processes, including enzyme function, antioxidant defense, and energy production. Manganese is essential for the activity of various enzymes, such as those involved in metabolism and cellular signaling.

Calcium, while also important, is needed in much larger quantities, particularly for bone health and various cellular functions. Iron, another essential element, is required in larger amounts than trace elements, as it is crucial for oxygen transport and energy metabolism. Sodium is essential for fluid balance and nerve function, but it is also not classified as a trace element, as it is required in larger amounts.

Overall, manganese stands out among the options as it is specifically required in microgram quantities, fitting the definition of a trace element.