Answer :

[tex]The\ perfect\ scuare:(a+b)^2=a^2+2ab+b^2\\------------------\\\\x^2+2x\to a=x;\ 2ab=2xb\to b=1\\\\therefore\\\\x^2+2x\cdot1+1^2=x^2+2x+\boxed{1}\leftarrow\boxed{A}[/tex]