Final answer:
The type of chemical used in preparing specimens for light microscopy that immobilizes macromolecular material and maintains cell structure is a fixative (option B) . Fixatives like formaldehyde and glutaraldehyde are used for this purpose.
Step-by-step explanation:
In the context of preparing specimens for light microscopy, the type of chemical that rapidly penetrates the cell membrane and immobilizes all of its macromolecular material while maintaining cell structure as close as possible to that of the living state is known as a fixative. Fixatives such as formaldehyde (formalin) and glutaraldehyde are used to preserve cellular and tissue architecture by denaturing proteins, stopping biochemical reactions, and stabilizing cell structures.
Fixation is an essential step in sample preparation for microscopy as it kills microorganisms, stops their metabolism, and preserves the integrity of cellular components for accurate observation under a light microscope. For certain types of specimens which are delicate and might be altered or destroyed by heat, chemical fixation is the preferred method over heat-fixing.
Therefore, the correct answer to the question is B) fixatives.