The tensile stress in a conical rod, having diameter \(\mathrm{D}\) at bottom, \(\mathrm{d}\) at top, length \(\mathrm{l}\) and subjected to tensile force \(\mathrm{F}\), at distance \(\mathrm{x}\) from small end will be:

(a) \(\frac{4 F}{\pi D^{2}}\)

(b) \(\frac{4 F}{\pi d^{2}}\)

(c) \(\frac{4 F l^{2}}{\pi((D-d) x+l d)^{2}}\)

(d) \(\frac{4 F}{\pi(D-d)^{2}}\)