Which is necessary for visualizing latent stains of blood, semen, or urine at a crime scene
a) reflective screens
b) ultraviolet light source
c) external light source
d) flash unit

B) ultraviolet light source

Explanation:

An ultraviolet light source is used to locate bodily fluids like semen, sweat, saliva and vaginal fluids at the scene of crime. These fluids emit their own natural fluorescence in the presence of ultraviolet light which are otherwise remain hidden and undetected at the scene of crime by naked eyes. This technique helps in their detection, collection and identification.

c. magnitude and direction