Chapter 1 - Measurement and Problem Solving - Learning Path Questions and Exercises - Multiple Choice Questions - Page 26: 7

(a) Nanometer is an unit for measuring length .it is an one billionth of meter , 10^-9 meters The correct answer is option (a)

A nanometer (nm) is one billionth ($10^{9}$) of a meter. . Nanotechnology involves the manufacture or building of things on an atomic scale. So, the option (a) is correct.