In which option is the expression not a polynomial because of the variable power?

Correct answer: x^(-1/3) + 7

For a polynomial in x, every exponent of x must be a non-negative integer. Option C contains x^(-1/3), whose exponent is both negative and fractional. It therefore fails the defining rule and is not a polynomial. It can also be interpreted as a reciprocal involving a fractional root, which confirms the same issue. Option A has powers 5, 1, and 0, all allowed. Option B has powers 2, 1, and 0; pi is simply a real coefficient and does not cause any problem. In option D, x^0 equals 1, so the expression simplifies to 11 - 5x, a valid linear polynomial. Therefore the variable power in option C is the reason it is excluded.