Correct Answer: 71

Solution : Let's check the options –
First option: 42; 42 is divisible by 1, 2, 3, 6, 7, 14, 21, 42. It is not a prime number.
Second option: 84; 84 is divisible by 1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42, 84. It is not a prime number.
Third option: 91; 91 is divisible by 1, 7, 13, 91. It is not a prime number.
Fourth option: 71; 71 is divisible by only 1 and 71. It is a prime number.

The fourth option does not follow a similar pattern as the other options. Hence, the fourth option is correct.