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)x2+1/2 x (9)(1)2
or 3+24=200x2+9
or x=√(18/200)
or x=0.3 m
x=30 cm