Let breadth = 4x, then length will be 5x.
According to question,
length × breadth =20
So 20x2 = 20
So x=1
Then breadth=4 and length =5 & their difference will be 1 cm.
Now, someone might think that option (a) is correct i.e. 1 m.
But the correct option is None of these (because all the other options are in meters).