Resistivity (Ñ) of a material is a property which describes its resistance to electric current flow. It quantifies how strongly a given material opposes the flow of electric current. Its unit must be a measure of resistance times a distance.

Option 1 (ohm) is not correct, because the ohm is a unit of resistance and doesn`t account for the material`s dimensions.

Option 2 (ohm/cm) is also incorrect. Resistivity must account for both path length and cross-sectional area, so simply dividing resistance by length doesn`t work.

Option 3 (ohm-cm) is the correct answer. Resistivity is commonly expressed in ohm-cm, which takes into account both the length and cross-sectional area of the material.

Option 4 (ohm-cm^2) is incorrect, because this would account for the cross-sectional area twice, which does not align with the definition of resistivity.

Therefore, option 3 or ohm-cm is the correct unit of resistivity.