The OP knows what this answer means.
NObody could be dumb enough to ask a question like this and need the answer. If you are the exception to my rule, please don't breed.
Stupid is as stupid does, go with d, or c, if you want. The answer is "a".
a.)Rhinos being abducted by aliens to experiment on them.
b.)Rhinos being hunted for sport.
c.)Rhinos being poached for their horns.
d.)Rhino habitats being destroyed by human settlers.
I am kind of leaning towards d because not many people live where rhinos do but I think c is also a legitimate answer to this question