b. 4 cm
a square has all the same sides so basically 4 times a number will get you 16.
it isn't hard to notice it.
10 x 4 = 40, so d aint it.
8 x 4= 32 si a. isn't it.
9 x 4 = 36 so b isn't it either.
4 x 4 = 16, so that make sense.
i didn't know how else to describe it but process of elimination is still easy for a problem like this.