Which process is characterized by thickening or congealing a substance?

The process referred to in this question is coagulation, which involves the thickening or congealing of a substance, typically in a biological context. Coagulation is most commonly associated with blood, where it refers to the transformation of liquid blood into a gel-like substance, forming a blood clot. This process is essential for stopping bleeding and begins with a complex cascade of reactions involving platelets and proteins in the blood.

Coagulation can also occur in other substances, such as when proteins denature and aggregate to form a solid or semi-solid mass when exposed to heat or acidic environments, as seen in cooking (for example, the coagulation of egg whites when they are cooked).

The other terms in the question refer to different processes. Contamination typically refers to the introduction of impurities or unwanted substances. A spore is a reproductive unit capable of developing into a new individual, often associated with fungi or bacteria. The portal of entry relates to the path by which pathogens enter the body. Understanding these distinctions helps clarify why coagulation is the process specifically identified in this question.