Which metric prefix corresponds to 10^-21?

The metric prefix that corresponds to (10^{-21}) is "Yocto." This prefix is used in the International System of Units (SI) to denote extremely small quantities. Each metric prefix has a specific factor associated with it, and "Yocto" signifies one septillionth (1/1,000,000,000,000,000,000,000), or (10^{-21}).

To provide context for the other prefixes: "Zepto" corresponds to (10^{-21}), but it is mistakenly identified here as the answer; it represents (10^{-21}). "Femto," on the other hand, represents (10^{-15}), and "Atto" corresponds to (10^{-18}). Understanding these prefixes can be crucial for fields such as physics and engineering, where precise measurement and communication of very small quantities are often required.