Final answer:
Proteins, when exposed to certain stressors like heat and chemicals, undergo denaturation, which alters their structures and causes loss of function. Therefore, the correct option is d.
Step-by-step explanation:
Once degradation begins, proteins, unlike other molecules, undergo a process known as denaturation. This is a change in the structure of a molecule through physical or chemical means. When they are exposed to factors such as extreme heat, acids, bases, or certain other substances, proteins will denature, meaning that the tertiary structure and possibly secondary and quaternary structures of the protein are altered. Denatured proteins lose their functional shape and are no longer able to carry out their jobs. For example, the curdling of milk when acidic lemon juice is added is an everyday example of protein denaturation. Therefore, the correct answer to the question is 'D. denature'. Denaturation can affect a protein's primary structure, secondary structure, tertiary structure, and quaternary structure.