Which of the following functions cannot use the second derivative test to determine if x=4x is the location of a relative minimum or relative maximum?

a) y=13x³−4x²+16x
b) y=xe⁻ˣ/⁴​
c) y=−cos⁡(x−4)y=−cos(x−4)
d) y=−x²+8xy=−x2+8x