Which expression is a polynomial?

A. 9x^7y^-3z

B. 4x^3-2x^2+5x-6+1/x

C. - 13

D. 13^-2

I believe the answer is C.

A polynomial cannot contain a negative number raised to the power (eliminating A. and D.) and a polynomial cannot contain any division by a variable (eliminating B.)

C. is what I would consider a polynomial. It contains one term, and does not break any rules.