ANSWER
b. y = 5·(0.52)^x
Step-by-step explanation
In the graph we can see that when x = 0, the function is 5 - this is the y-intercept. Therefore, the coefficient must be 5, so it's either a or b.
Then, the growth/decay factor must be less than 1, because it's a decay as seen in the graph. Therefore, the correct function is option b: