The term that refers to bloodstains that are not visible to the unaided eye is latent bloodstains (option C).
Latent is a term used to describe a substance that is dormant or hidden until circumstances are suitable for development or manifestation.
Latent bloodstains are those bloodstains that are not visible to the unaided eye and can only be viewed using a microscope.
Scientists and laboratory personnel can use chemical reagents such as Luminol to find and photograph latent bloodstains.