To determine which option cannot be used to measure length, let's briefly review what each unit represents:

• Fermi: A fermi (or femtometer) is equal to $10^{-15}$ meters and is commonly used in nuclear and particle physics to measure very small distances, making it a unit of length.
• Debye: A debye is not a unit of length but rather a unit of electric dipole moment. Thus, it cannot be used to measure length.
• Micron: A micron (or micrometer) is equal to $10^{-6}$ meters and is widely used in various scientific fields to measure lengths at the microscopic scale.
• Light year: A light year is the distance light travels in one year, approximately $9.46 imes 10^{15}$ meters, and is a valid unit of length used in astronomy.

Since the question asks for the option that cannot measure length, the correct answer is: