The correct option is C 9
Consider the expression x2+6x.
⇒x2+2×x×3
It is in the form of a2+2ab
By adding b2 we get the identity
a2+2ab+b2=(a+b)2 that represents a perfect square.
⇒x2+2(x)(3)+32
=(x+3)2=x2+6x+9
is a perfect square.
Thus, we have to add 9 to the expression x2+6x so that the resultant is a perfect square.