Answer:
Option C. 4.2 units
Explanation:
Diagonals of a square intersect each other at 90°.
Therefore, From the right angle triangle formed at the center will be,
(Length of square)² = 3² + 3² [By Pythagoras theorem}
Length of the square =

= 4.24 units
≈ 4.2 units
Option C is the answer.