(x+y)^3(x+y)^3= (x^3+3xy^2+3x^2y+y^3)(x^3+3xy^2+3x^2y+y^3)=
x^6 + 3x^4y^2 +3x^5y+ x^3y^3 + 3x^4y^2 + 9x^2y^4 + 9x^3y^3 + 3xy^5 + 3x^5y + 9x^3y^3 + 9x^4y^2 + 3x^2y^4 + x^3y^3 + 3xy^5 + 3x^2y^4 + y^6=
x^6+y^6+ 15 x^4y^2+ 6x^5y + 20 x^3y^3 + 15x^2y^4+6xy^5
is the first option