Answer:
C. Its physical structures
Step-by-step explanation:
A dichotomous key is defined as a tool used to differentiate between objects and organisms. A dichotomous key consist of series of question related to the species and at the end gives the scientific name of the species.
The characteristic that is most helpful when identifying organism using a dichotomous key is "Its physical structures". for example: if a person want to identify a tree, a dichotomous key ask about the physical structures such as if the tree has needles or leaves.
So, physical structures are most helpful while using a dichotomous key.