It depends on your teacher, it should be A. It could be B if that’s what your teacher wants but I would go with A.
It is an explosive cell that is used for protection from predators or prey hunting by the species of the phylum Cnidaria
Human,increase population and Overharvesting
Answer: The length of the cell in millimetres is 0.0015.
Explanation:
Given conversion :

Thus 
Given : Length of the cell = 1.5 micrometers 
To find: Length of the cell in milllimeters (mm)
Length of the cell in milllimeters (mm) = 
The length of the cell in millimetres is 0.0015.