3x3−50x is a binomial with highest degree 2. Hence, 3x3−50x is a cubic polynomial.
2x55 has only one term. Hence it is a monomial.
4x3+18x2+5x+6 is a polynomial with one term. Hence, it is a Quadrinomial.
x2−55x+5 is a trinomial with highest degree as 2. Hence, it is a quadratic polynomial.