The correct answer is that strain is dimensionless.
Strain is always a ratio of two similar quantities, such as change in length to original length, change in volume to original volume, or change in shape angle to the original configuration.
Whenever a physical quantity is defined as the ratio of two quantities having the same dimensions, the dimensions cancel out and the result carries no dimensions.
This is the same reason relative density and refractive index are also dimensionless, they too are ratios of similar quantities.
So strain has no unit like metre or second attached to it, it is just a pure number describing how much a body has deformed relative to its original size.