Four numbers have been given, out of which three are alike in some manner and one is different. Select the number that is different from the rest.

Let's analyze the options

a) 81→ composite number 

b) 61→  prime numbers 

c) 83 → prime numbers ​

d) 47 → prime numbers ​

So, 81 is the different one because it is composite, while the others are prime.

Thus, correct option is (a).