Which of the following prefixes indicates a factor less than one?

The prefix that indicates a factor less than one is "deci." In the metric system, deci represents one-tenth of a unit, or (10^{-1}). This means that when you see "deci" in front of a unit, it signifies that you are dealing with one-tenth of that unit. For instance, one decimeter (dm) is equal to 0.1 meters.

In contrast, the other prefixes mentioned all indicate multipliers greater than one. "Hecto" signifies a factor of one hundred, "kilo" represents a factor of one thousand, and "mega" denotes a factor of one million. These prefixes are used to express larger quantities, hence they do not fit the criteria for indicating a factor less than one. Understanding these prefixes is crucial for accurately interpreting measurements in scientific and technical contexts.