Out of the four numbers listed, three are alike in some manner and one is different. Select the odd one.

2126, 9819, 7568, 8423

Now, we check each options.

Option (a): 2126 → Composite Number

Option (b): 9819 → Composite Number

Option (c): 7568 → Composite Number

Option (d): 8423 → Prime Number

So, 8423 is the only prime number.

Thus, correct option is (d).

(Note:→ Other logic- product of 1st and last digits give the middle number)