Answer:

Option C. 4.2 units

Step-by-step 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 = sqrt{9+9}=sqrt{18}

                                   = 4.24 units

                                   ≈ 4.2 units

Option C is the answer.