How is conductivity of a material defined?

• A. <a href="https://www.sanfoundry.com/wp-content/uploads/2017/12/engineering-materials-metallurgy-questions-answers-electrical-properties-q2-2.png"><img alt="The conductivity of a material defined - option a" class="size-full wp-image-183911" height="42" src="https://www.sanfoundry.com/wp-content/uploads/2017/12/engineering-materials-metallurgy-questions-answers-electrical-properties-q2-2.png" width="33"/></a>
• B. <a href="https://www.sanfoundry.com/wp-content/uploads/2017/12/engineering-materials-metallurgy-questions-answers-electrical-properties-q2-3.png"><img alt="The conductivity of a material defined - option b" class="size-full wp-image-183912" height="42" src="https://www.sanfoundry.com/wp-content/uploads/2017/12/engineering-materials-metallurgy-questions-answers-electrical-properties-q2-3.png" width="20"/></a>
• C. <a href="https://www.sanfoundry.com/wp-content/uploads/2017/12/engineering-materials-metallurgy-questions-answers-electrical-properties-q2-1.png"><img alt="The conductivity of a material defined - option c" class="size-full wp-image-183910" height="42" src="https://www.sanfoundry.com/wp-content/uploads/2017/12/engineering-materials-metallurgy-questions-answers-electrical-properties-q2-1.png" width="33"/></a>
• D. <a href="https://www.sanfoundry.com/wp-content/uploads/2017/12/engineering-materials-metallurgy-questions-answers-electrical-properties-q2-4.png"><img alt="The conductivity of a material defined - option d" class="size-full wp-image-183913" height="42" src="https://www.sanfoundry.com/wp-content/uploads/2017/12/engineering-materials-metallurgy-questions-answers-electrical-properties-q2-4.png" width="22"/></a>