Answer
$a^2b^2+2abd+d^2$
Work Step by Step
RECALL:
$(x+y)^2=x^2-2xy+y^2$
Use the formula above to have:
$(ab+d)^2
\\=(ab)^2+2(ab)(d) + d^2
\\=a^2b^2+2abd+d^2$
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