Number Classification

The series follows the pattern n²-1 where n starts from 3: 3²-1=8, 4²-1=15, 5²-1=24, 6²-1=35, 7²-1=48 (not 50), 8²-1=63, 9²-1=80. So 50 is incorrect; it should be 48. The pattern generates consecutive squares minus 1.

The series contains prime numbers (7, 11, 13, 17, 19, 25, 29), but 21 is not a prime number since 21 = 3 × 7. All other numbers are prime. So 21 is the odd one out.

Only in option 1 is the arrangement of the digits different as compared to the rest of the options.

In all options except B, the product of the two numbers equals 126: 14×9=126, 21×6=126, and 42×3=126. Option B (17, 8) is the odd pair because 17×8=136, not 126.

In options A, B, and C, the difference between the digit sums of the two numbers equals 4: (3+4)-(1+2)=4, (4+3)-(3+0)=4, (5+2)-(2+1)=4. Option D is odd because (6+2)-(1+9)=8-10=-2, which has an absolute value of 2, not 4.

Options A, B, and C follow the pattern: x, x², x³ - where 3→9→27 (3, 3², 3³), 5→25→125 (5, 5², 5³), and 7→49→343 (7, 7², 7³). Option D is odd because 529 is 23², not 9³ (which would be 729).

In options A, B, and D, the digit sums of both numbers in each pair are equal: 18→9, 45→9; 23→5, 14→5; 36→9, 27→9. Option C is odd because the digit sums differ: 29→11, 82→10.

Options A, B, and C are all divisible by 20 (ending in 20 or 40): 8540÷20=427, 6740÷20=337, 5420÷20=271. Option D (3412) is odd because it is not divisible by 20 - 3412÷20=170.6, and it doesn't follow the same divisibility pattern.