Answer: Option (A) is the correct answer.
Step-by-step explanation:
A transition element is defined as the element which has incompletely filled d-orbital.
For example, atomic number of chromium is 24 and its electronic configuration is
.
Also, sometimes the s-orbital is completely filled and sometimes it is half-filled.
Hence, we can conclude that a single factor that classifies an element as a transition metal is that the highest occupied orbital is a "d" orbital.