When the angle of plane is just the block just begins to slide down $\tan \theta=\mu$

Also the force needed to push a mass upward $= mg \sin \theta+\mu mg \cos \theta$

acceleration $=\large\frac{Force}{mass}$

$\qquad= \large\frac{mg \;\sin \theta+\mu \cos \theta}{m}$

$\qquad= g \sin \theta+ \mu g \cos \theta$

$\qquad= g \sin \theta+ \tan \theta g \cos \theta$

$\qquad= g \sin \theta+ g \sin \theta$

$\qquad= 2g \sin \theta$

Hence b is the correct answer