**Correct option is (1)**

**Explanation:**

At maximum extension velocity of both the blocks will be same. Let v be the common velocity of the blocks (towards right). From conservation of linear momentum.

(3+6)v=6×2−3×1=9

or v=1m/s

Now let x be the maximum extension in the spring.

From conservation of mechanical energy

1/2 x (3)(1)^{2}+1/2 x (6)(2)^{2}=2(200)x^{2}+1/2 x (9)(1)^{2}

or 3+24=200x^{2}+9

or x=√(18/200)

or x=0.3 m

**x=30 cm**