Inside a horizontally moving box, a man finds that when an object is placed on a horizontal smooth table and released; it moves with an acceleration $10 m/s^2$ In this box if 1 kg body is suspended with a light string, the tension in the string in equilibrium position $(g=10 m/s^2)$

Question

$$(a)\;10 \;N \quad (b)\; 10\;\sqrt 2 \;N \quad (c)\;20 \;N \quad(d)\;zero$$