Out of the number pairs listed below, three are alike in the way they are related to each other. Select the odd one.

Logic: For each pairs, subtract 1 from the second number and then multiply it by the second number to get the first number.

Explanation:​ 

(a) 20 : 5$$\rightarrow$$5 $$\times$$ (5 - 1) = 5 $$\times$$ 4 = 20​

(c) ​56 : 8$$\rightarrow$$8 $$\times$$​ (8 - 1) = 8 $$\times$$​ 7 = 56​

(d) ​42 : 7$$\rightarrow$$7 $$\times$$​ (7 - 1) = 7 $$\times$$​ 6 = 42

So, only the second option does not follow the same pattern.

Thus the correct answer is (b).