Which of the following is the smallest unit?

1. Millimetre
2. Angstrom
3. Fermi
4. Metre

Fermi

Reason — Fermi is 10^-15th part of a metre. i.e., 1 fermi = 10^-15 m, hence it is the smallest unit.

Angstrom = 10^-10th part of a metre.
Millimetre = 10^-3th part of a metre.