We have to check each optuions:
1, 1, 2
Let the largest number be the hypotenuse
Using pythagoras theorem
opp² + adj² = hyp²
1² + 1² = hyp²
1 + 1 = hyp²
2 = hyp²
hyp = √2 = 1.4
So this can't be right triangle
Let's check the next option
5, 6, 12
5² + 6² = hyp²
25 + 36 = hyp²
61 = hyp²
hyp = 7.8
This can't be a choice for a right-triangle
Next,
3, 5, 8
3² + 5² = hyp²
9 + 25 = hyp²
34 = hyp²
hyp = 5.8
This can't be a choice for a right-triangle
Next,
10, 24, 26
10² + 24² = hyp²
100 +576 = hyp²
676 = hyp²
hyp = 26
This is a choice for the length of the sides of a right triangle
Hence option D is the correct option