Step 1: Understanding the Concept
The cylinder floats at the interface, so its weight equals the total upthrust from the two liquids. Atmospheric pressure acts equally on the top and bottom faces and adds no net force.
Step 2: Set up
From the figure the upper liquid has density \(d\) and the lower one \(2d\). The length \(\dfrac{L}{4}\) is in the lower liquid and \(\dfrac{3L}{4}\) in the upper one.
Step 3: Balance of forces
Let the cylinder density be \(\rho\) and its cross-section \(S\):
\(\rho LSg=\dfrac{3L}{4}S\,d\,g+\dfrac{L}{4}S\,(2d)\,g\)
Step 4: Solve
\(\rho=\dfrac{3d}{4}+\dfrac{2d}{4}=\dfrac{5d}{4}\), option (A).
Final Answer:
The density of the cylinder is \(\dfrac{5d}{4}\).
\[ \boxed{\text{(A) }\dfrac{5d}{4}} \]