Which of the following is a common mounting medium used for histological slides?

Canada balsam is a traditional and widely used mounting medium for histological slides. It is a natural resin derived from the Abdens douglassii tree and is favored for its optical clarity and ability to preserve the integrity of the specimens over time. This medium allows for proper light transmission, which is critical when viewing specimens under a microscope.

Cedar wood oil is more commonly used as a clearing agent rather than a mounting medium. It can enhance the refractive index of specimens but is not suitable for long-term slide storage because it is not permanent.

Formalin is primarily used as a fixative rather than for mounting. It preserves tissue morphology but does not provide the optical qualities required for mounting specimens on slides.

Alcian blue is a histological stain used to identify acidic mucins in tissues and is not a mounting medium. Its purpose is to add color to specific components of tissue, rather than to serve as a medium for slide preparation.

Thus, Canada balsam stands out as the correct answer due to its long-standing use and suitability as a mounting medium in histology.