4. Which of the following is an example of a polynomial function?

a.
[tex]f(x) = \frac{3}{x {}^{3} } + 5x - 1[/tex]
b.
[tex]f(x) = 4 {x}^{ \frac{3}{2} } - 3 {x}^{2} [/tex]
c.
[tex]f(x) = \sqrt{15} x - 2 {x}^{3} [/tex]
d.
[tex]f(x) = {x}^{2} + \sqrt{ {2x}^{} } - 7[/tex]
​