The smallest metric prefix out of those listed is B) micro. Here's why:

* Micro (µ): Represents 1/1,000,000 (one millionth)

* Milli (m): Represents 1/1,000 (one thousandth)

* Centi (c): Represents 1/100 (one hundredth)

* Deci (d): Represents 1/10 (one tenth)

Since a millionth is smaller than a thousandth, a hundredth, or a tenth, micro is the smallest prefix.