a
Young's modulus \(\quad Y =\frac{\text { Stress }}{\text { Strain }}\)

As we know, strain is a dimensionless quantity whereas stress is defined as force per unit area.

Stress has units as \(Nm ^{-2}\) (SI unit), Dyne \(cm ^{-2}\) (cgs unit) or Pascal \(( P a)\).

As Young's modulus has same unit as that of stress, thus \(Nm ^{-1}\) is not a unit of Young's modulus.