Answer:

Explanation:
Remember that a maximum value (global) of a function is the bigest value that the function takes.
For see the values of the functions you can see its graphs.
I have uploaded the graphs of the functions in the question and you can see it, and then convince yourself why the answer is y=-5x^4.
Also you have the fact that the functions of the form
have a maximum value only if
is an even natural number and k is a negative real number.