Explanation

(a) We can realise the situation as shown. Maximum retardation
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$a = \frac{f_{k} - mg\sin\theta}{m}$
$= \frac{\mu_{k}\ mg\cos{\theta - mg\sin\theta}}{m}\ $
$= \frac{\frac{1}{\sqrt{3}}mg\cos{30{^\circ} - mg\sin{30{^\circ}}}}{m}\ (\because\ \theta = 30{^\circ})$
$= \frac{\frac{1}{\sqrt{3}}g \times \frac{\sqrt{3}}{2} - g \times \frac{1}{2}}{1} = 0$
Hence, under the effect of kinetic friction between block and inclined plane, acceleration of block is zero.