The Hoop stress developed in the thin cylinders is given by:
(where P = Internal pressure, d = Internal diameter and t = wall thickness)

Correct option is A

Hoop stress (or circumferential stress) in a thin walled cylindrical pressure vessel is the stress exerted in the circumferential direction when the cylinder is subjected to internal pressure. The formula for hoop stress is given by:
σh = Pd/2t
where:
(P) = Internal pressure
(d) = Internal diameter of the cylinder
(t) = Wall thickness of the cylinder
This relationship is crucial for ensuring the structural integrity of pressure vessels, as it helps in determining the required wall thickness for safe operation under a given internal pressure.
Thus, the correct formula for hoop stress in a thin-walled cylinder is Pd/2t