Tanning of leather is the: 

The process of tanning involves removing the excess fat from the skin or hide to prepare it for use. Because the fat in the dead skin will obstruct the manufacturing process, it must be appropriately managed. The tanning procedure does that.

The skin is wet when it is extracted, and if it is not chemically maintained or processed, it will decay and become useless. Therefore, the leather needs to be adjusted in order for it to fit and be useful. Chemical skin manipulation occurs during tanning. Positively charged colloidal particles are found in animal skin (hide), whereas negatively charged colloidal particles are found in tanning. Mutual coagulation occurs when the skin, or you might say hide, is submerged in tanning solution, and as a result, the leather hardens. In some circumstances, colouring and coagulation are carried out concurrently. The most common definition of tanning is the chemically induced coagulative hardening of leather.

Hence, option A is correct.