Which expression is a perfect-square trinomial and can be written in the form \((x+a)^2\)?

Correct answer: \(x^2+10x+25\)

Direct answer: A, \(x^2+10x+25=(x+5)^2\). The basic identity is \((x+a)^2=x^2+2ax+a^2\). The constant term 25 has square root 5, so the middle term required for a positive square is \(2\cdot x\cdot5=10x\). Both conditions are satisfied, and therefore the expression is \((x+5)^2\). Option A is correct. Option B has the same first two terms but its constant is 20 rather than 25, so it cannot be \((x+5)^2\). Option C is a difference of squares, not a trinomial, and it factors as \((x-5)(x+5)\). Option D is a sum of two squares and has no middle term, so it does not match the identity. Memory cue: take the square root of the constant and double it to predict the coefficient of \(x\).