Answer:
D. 3, 4, 5
Explanation:
Pythagorean theorem :

If the given options satisfy the theorem .So, they form a Pythagorean triple.
Hypotenuse is the longest Side
A)7, 8, 9
Hypotenuse = 9




Hence not a Pythagorean triple.
B)1,2,3
Hypotenuse = 3




Hence not a Pythagorean triple.
C) 4,5,6
Hypotenuse = 6




Hence not a Pythagorean triple.
D)3,4,5
Hypotenuse = 5




Hence a Pythagorean triple.
Hence Option D represents side lengths that form a Pythagorean triple