Explanation

(b) Frictional force is independent of the area of surfaces in contact, as long as normal reaction between the two bodies in contact remains the same
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When an iron block of mass $m$ with surface area $A$ is placed on an inclined plane inclined at an angle $\theta$, it will experience a frictional force $f$ as shown in figure (i)
$f = \mu R = \mu mg\cos\theta$ …(i)
When an iron block of mass $m$ with surface area $2A$ is placed on an inclined plane inclined at an angle $\theta$, it will experience a frictional force $f''$ as shown in figure (ii)
$f^{''} = \mu R = \mu mg\cos\theta$ …(ii)
From (i) and (ii), we get $f^{''} = f$