A small mass is hung from the ceilling of a train compartment. The train moves up an incline of $30 ^{\circ}$ with horizontal. Acceleration of train up the incline is $a=\large\frac{g}{2}.$ The angle with which the string supporting the mass makes with normal to ceiling in equilibrium

Question

$$(a)\;30 ^{\circ} \quad (b)\;\tan ^{-1} \bigg(\frac{2}{\sqrt 3}\bigg) \quad (c)\;\tan^{-1}\bigg(\frac{\sqrt 3}{2}\bigg) \quad (d)\;\tan ^{-1} 2$$