C

Step 1: Understanding Stokes' Law
Stokes' law describes the force of viscosity acting on a sphere moving through a viscous fluid. It is expressed as:
$$ F = 6 \pi \eta r v $$
where $\eta$ is the viscosity, $r$ is the radius of the sphere, and $v$ is its velocity. The corresponding reason is [B] force of viscosity.

Step 2: Equation of Continuity
This principle states that when a fluid moves through a pipe, the mass flow rate must remain constant. Thus, it connects to the [C] Conservation of Mass.

Step 3: Bernoulli's Theorem
Bernoulli's theorem states that for an incompressible, frictionless fluid, the sum of the pressure energy, kinetic energy, and potential energy per unit volume is constant along any streamline. This corresponds with [D] Conservation of energy.

Step 4: Velocity of Efflux
The velocity of efflux is derived from Torricelli’s theorem, which relates the speed of fluid flowing out of an orifice to the height of the fluid above it, reflecting [A] Surface potential energy.

Based on the analysis:

• (i) Stokes' law -> [B]
• (ii) Equation of continuity -> [C]
• (iii) Bernoulli's theorem -> [D]
• (iv) Velocity of efflux -> [A]

Therefore, the correct match for (iii) Bernoulli's theorem is [D] Conservation of energy.