A uniform cylindrical rod of cross-sectional area S is placed on a...

Consider a cross-section of the rod at any point along its length.

Since the rod is in equilibrium under the action of two equal and opposite forces $F$ at its ends, the net force on any part of the rod is zero.

The internal restoring force (tension) $T$ developed at any cross-section must balance the external force $F$ applied at the ends.

Thus, $T = F$.

Tensile stress is defined as the internal restoring force per unit cross-sectional area.

Tensile stress = $\dfrac{T}{S} = \dfrac{F}{S}$.

Answer: $\dfrac{F}{S}$