So to find the degree of a polynomial, just count up all the exponents in the term with the greatest amount. (Remember that variables without an exponent are assumed to be in the first power)
[tex] 2x^2y=2+1=3\\ -8xy^3=1+3=4\\ 5xy=1+1=2 [/tex]
Since the term with the largest total amount of exponents is -8xy^3 with 4, the degree of this polynomial is 4th degree, or B.
