Find out the number which is least like the other numbers in the following…

Concept: Each term in this series is a perfect square. In this classification pattern, a set of squares is built from consecutive prime numbers; the term whose square root is not a prime number is the one that does not fit with the rest.

1. 4 = 22, and 2 is a prime number.

2. 9 = 32, and 3 is a prime number.

3. 25 = 52, and 5 is a prime number.

4. 49 = 72, and 7 is a prime number.

5. 81 = 92, and 9 = 3 × 3, so 9 is a composite number, not a prime number.

6. 121 = 112, and 11 is a prime number.

7. 169 = 132, and 13 is a prime number.

Cross-check: Reading the square roots in order — 2, 3, 5, 7, 9, 11, 13 — six of the seven values (2, 3, 5, 7, 11, 13) are exactly the first six prime numbers, while 9 is the only composite number among them. A competing reading does exist: the roots 3, 5, 7, 9, 11, 13 are consecutive odd numbers, and that reading would instead single out 4, the square of the even number 2. The prime-square reading is the one the official answer key adopts, because its run of roots begins at the very first prime, 2, and covers the primes without any gap. So 81 is the single term that breaks the pattern followed by every other term in the series.

Therefore, 81 is the number that is least like the other numbers in the series.