Correct option is (A) 3360 N
If T is the tension in thread and a be acceleration of 100kg block
For the 10 kg block kept on the 100 kg block (In the 100 kg block frame of reference,
10a - T = 10 x 2 ......(1)
For the 20 kg block in vertical direction,
T - 20g = 20 x 2
⇒ T - 20 x 10 = 20 x 2 .......(2)
Adding the above two equations,
⇒ 10a = 3 x 20 + 20 x 10 = 260
a = 26ms-2, T = 240 N
The applied force F will be responsible for the horizontal acceleration of 100 kg and 20 kg block.
Therefore,
F - T = 120 a
⇒ F = 3360 N