What is the approximate size of a micron?

The correct choice accurately identifies a micron, which is commonly defined as one millionth of a meter. This unit of measurement, also referred to as a micrometer, is widely used in various fields such as engineering, biology, and materials science to measure small distances and sizes.

In practical terms, a micron is equivalent to 0.001 millimeters; therefore, understanding the relationship between these units is crucial when working with measurements in instrumentation and controls. The other options do not accurately describe the size of a micron, further confirming that the correct response is indeed based on the widely accepted scientific definition that places a micron at one millionth of a meter.