A U-tube open to the atmosphere at both ends is partially filled with water of density \(\rho_w\). Oil of density \(\rho_o\) (where \(\rho_o < \rho_w\)) is poured into the left arm until it forms a column of height \(L\). The interface between the oil and water in the left arm is found to be a vertical distance \(d\) below the water surface in the right arm. Which of the following expressions correctly represents the density of the oil?