Forces Acting on Upward Moving Bubble in Liquid

To determine the forces acting on the bubble as it moves upwards through the liquid, let us analyze the physical interactions between the bubble and its surroundings:

1. Force due to Liquid Pressure (Buoyancy Force):
   The pressure of the surrounding liquid acts perpendicular to every element of the bubble's surface. The net force resulting from the integration of this hydrostatic pressure over the entire closed surface of the bubble is, by definition, the buoyant force ($\vec{F}_B$):
   $\vec{F}_B = -\oint_S P \, d\vec{A} = \rho_l V g \, \hat{k}$
   Since the question asks for the forces acting on the bubble besides the buoyancy force, including the "force due to the pressure of the liquid" would be redundant (double-counting the same physical interaction).

2. Force of Gravity:
   The gas inside the bubble has mass $m = n M$, where $M$ is the molar mass of the gas. Hence, it experiences a downward gravitational force:
   $\vec{F}_g = -mg \, \hat{k}$

3. Force due to Viscosity of the Liquid:
   As the bubble moves upwards through the fluid with a relative velocity $\vec{v}$, the surrounding liquid exerts a resistive viscous drag force $\vec{F}_v$ on the bubble opposing its motion (acting downwards):
   $\vec{F}_v = -6\pi \eta r \vec{v}$ (or more generally, a hydrodynamic viscous force).

Therefore, besides the buoyancy force, the forces acting on the upward-moving bubble are:

• The force due to gravity, and
• The force due to the viscosity of the liquid.

Hence, the correct option is (D).