b
(b)
Tensile stress $=\frac{\text { Tension at point }}{\text { Area }}$
Tension at distance $x$ from top would be the amount of force acting due to all the weight below it
$=$ Mass per unit length of rod $\times$ length of rod $+M g$ $=m \times(I-x) g+M g$
So Tensile stress $=\frac{m(I-x) g+M g}{A}$
