$1$ point possible $($graded$)$

Which of the following feature vectors $(x)$ produces the kernel

$K(x,x^{\text{'}})=(x)*(x^{\text{'}})=x_1{x}_1^{\text{'}}+x_2{x}_2^{\text{'}}+x_3{x}_3^{\text{'}}+x_2{x}_3^{\text{'}}+x_3{x}_2^{\text{'}}$

$(x)=[x_1,x_2,x_3]$

$(x)=[x_1+x_2+x_3]$

$(x)=[x_1,x_2+x_3]$

$(x)=[x_1+x_3,x_1+x_2]$