(a) a stone cannot be ethical (b) whereas human can be ethical

You didn't give the options. However, i'll try.
To me, a stone cannot be ethical because it cannot suffer. Indeed, the capacity for suffering must be satisfied before we talk about interest in a meaningful way. For instance, nothing we could possibly do for a stone could make a difference in its welfare. It don't have interest.  Whereas, we, humans have interest. Therefore we can be ethical because we are sentient beings that can be benifited or harmed. It's because we can experience pain as a result.
Hope this helps !
Photon