$24cm$

$26cm$

$23cm$

$28cm$

A. $24cm$

We know perimeter of square = 4(side)

So Side of first square = 40/4 = 10 cm

Side of second square = 32/4 = 8 cm

Area of third Square = 10*10 - 8*8

= 36 cm

So side of third square = 6 (because area of square = side*side)

$Perimeter=4*Side=4*6=24cm$