Resistivity is defined as:

Resistivity is fundamentally defined as the resistance of a material per unit length and unit cross-sectional area. This characteristic is intrinsic to the material and reflects how strongly it resists the flow of electric current. The concept allows for the comparison of different materials' ability to conduct electricity. By normalizing resistance based on geometric factors (length and cross-sectional area), resistivity provides a clear understanding of a substance's conductive properties independent of its physical dimensions.

The other definitions provided relate to broader concepts in electrical engineering. While the opposition to charge movement (the second option) and relationships between voltage and current (the third option) describe aspects of electrical behavior, they do not precisely define resistivity. The fourth option, which refers to the sum of voltages in a closed loop, pertains to Kirchhoff's voltage law and does not connect directly to the definition of resistivity. Thus, while all the concepts play essential roles in circuit theory, the definition of resistivity specifically concerns the characteristic of a material in terms of geometric resistance.