Right choice is (a) (x + y)^3
Easiest explanation: Using binomial expansions properties, x^4 + 4x^3y + 6x^2y + 4xy^3 + y^4 can be written as
= 4C0x^4y^0 + 4C1x^3y^1 + 4C2x^2y^2 + 4C3x^1y^2 + 4C4x^0y^4
= (x + y)^4
When divided by (x + y), we get (x + y)^3.