The correct options are
A (x+a)(x+b)=x2+(a+b)x+ab
C (a−b)2=a2−2ab+b2
D (x+y)2=x2+2xy+y2
An identity is an equality, which is true for all values of the variables in the equality.
Some of the algebraic identites are:
(x+a)(x+b)=x2+(a+b)x+ab
a2−b2=(a+b)(a−b)
(a−b)2=a2−2ab+b2
(x+y)2=x2+2xy+y2