Which one will replace the question mark?

In a linked-node number puzzle like this, every branch in the series is governed by the same arithmetic rule connecting the two numbers at the top to the single number at the bottom. Before applying it to the unknown branch, that rule must first be confirmed on the branches that are already fully solved.

1. First branch: (2)2 = 4 and (4)2 = 16.

2. Add the two squares: 4 + 16 = 20, which matches the given bottom value of the first branch, confirming the rule of squaring each top number and adding the squares.

3. Second branch: (3)2 = 9 and (9)2 = 81.

4. Add the two squares: 9 + 81 = 90, which again matches the given bottom value, confirming the rule holds across branches.

5. Third branch: (1)2 = 1 and (5)2 = 25.

6. Add the two squares: 1 + 25 = 26.

As an independent check, the same rule already verified twice ((2)2 + (4)2 = 20 for the first branch and (3)2 + (9)2 = 90 for the second) is being applied consistently rather than a new rule invented for the last branch, so the derived value of (1)2 + (5)2 = 26 is reliable; among the offered values only 26 equals (1)2 + (5)2.