1.

is not a polynomial, because the degree of second term is negative (-1);
2.

is a polynomial of 4th degree (the highest degree of all terms is 3+1=4);
3.

is a polynomial of 2th degree (the highest degree of all terms is 2);
4.

is not a polynomial, because the degree of second term is a fraction

;
5.

is a monomial with 1st degree;
6.

is not a polynomial, because here the variable x is placed in degree.