The correct option is A 6√2 units
Let the side of the square be s.
Given, the perimeter of the garden = 24 units
We know that, the perimeter of a square =4×s
⇒24=4×s (∵given the perimeter is 24)
Divide both sides by 4
⇒244=4×s4
⇒6=s
⇒s=6
Hence, the diagonal of the square =√2×s
=√2×6
=6√2 units
Hence, option (a.) is the correct choice.