Chin made an error in his chart showing the multiplication of the binomial by the trinomial (3x+y)(x^2+2y+4) which change can be made to correct the chart?

(a.) the expression 3x^3 should be 3x^2
(b.) the expression 6x should be 6xy
(c.) the expression x^2y should be x^2y^2
(d.) the expression 4y should be 4y^2