Answer:
C
Step-by-step explanation:
So, we started out with 2,400 black rhinos. 
And we know that they are declining at a rate of 10% per year. 
In other words, each subsequent year will only have <em>90%</em> of the previous year's rhinos. 
Therefore, our function is: 

We can do some elimination first. 
First, the starting point or y-intercept <em>must </em>be 2400 since that's the number of rhinos we started out with. 
Therefore, we can eliminate A and D. 
From the remaining choices B or C, we can technically also eliminate B. This is because from our function, we know it's exponential decay, and exponential decay is <em>not</em> a straight line. 
However, to double check, let's check the population after 10 years using our function. So: 

Use a calculator: 

So, after 10 years, the population should be around 840. 
B is definitely <em>not</em> 840 (at x=10). However, C is right around it. 
Therefore, we can conclude that C is the right answer. 
And we're done!