The power of a lens is calculated as the reciprocal of the focal length in meters. In this case, the focal length is -2m (negative because a concave lens is a diverging lens, and focal length is often taken as negative for diverging optical devices). Therefore, the power of the lens would be 1/focal length, which is 1/-2= -0.5 dioptre.

Option 1 indicates a positive power, which is incorrect because a concave lens always has a negative power. Option 3 also suggests a positive power, which is again incorrect. And finally, option 4 indicates a negative power but the magnitude does not match the correct calculation. Hence, only Option 2 matches the correct negative power of -0.5 dioptre for a concave lens with the given focal length.