What is defined as the removal of electrons from substances?

The correct choice is oxidation, which is the process defined as the removal of electrons from a substance. In a chemical context, oxidation refers to the loss of electrons during a reaction, which often occurs when a substance reacts with oxygen. This process is fundamental in many chemical reactions, particularly those involving energy transfer, such as combustion and respiration.

When a substance undergoes oxidation, it typically increases its oxidation state, and this often results in the substance becoming more positive (or less negative) in charge. Oxidation is also a key component of redox (reduction-oxidation) reactions, where one substance is oxidized while another is reduced (gains electrons).

Other concepts mentioned in the choices—such as reduction, filtration, and oxidative stress—are distinct processes with different definitions and implications. Reduction, for instance, is the gain of electrons by a substance, while filtration pertains to the physical separation of particles from liquids or gases. Oxidative stress refers to a condition arising from an imbalance between reactive oxygen species and antioxidants in the body, often linked to various diseases. Understanding oxidation is vital for comprehending various chemical processes and biological systems.