**Explanation:**

The efficiency of a sedimentation tank refers to its ability to separate solids from a liquid stream effectively. Increasing the surface area of the tank is the correct answer to increase the efficiency of a sedimentation tank. This can be explained through the following points:

**1. Sedimentation Process:**
The sedimentation process in a tank involves the settling of solid particles suspended in a liquid due to the force of gravity. As the liquid flows through the tank, the solid particles settle at the bottom, and the clarified liquid overflows from the top. The efficiency of the tank depends on the ability to separate the solids from the liquid effectively.

**2. Surface Area and Settling Time:**
Increasing the surface area of the tank allows for a larger contact area between the liquid and the settling solids. This increased contact area facilitates the settling process and reduces the settling time required for the particles to separate from the liquid. The longer the particles have to settle, the more efficient the tank becomes in removing them.

**3. Enhanced Particle-Fluid Interaction:**
When the surface area of the tank is increased, the particles have more opportunities to interact with the fluid. This interaction promotes the agglomeration and settling of the solid particles. In a tank with a larger surface area, the particles have to travel a shorter distance to reach the bottom, resulting in faster settling.

**4. Improved Clarification:**
Increasing the surface area also enhances the clarification process by allowing for a longer contact time between the liquid and any remaining suspended particles. This additional contact time allows for more effective removal of fine particles that may not settle under normal conditions.

**5. Design Considerations:**
When designing a sedimentation tank, the surface area can be increased by incorporating inclined plates, lamella settlers, or other mechanisms that increase the effective settling area without significantly increasing the tank's footprint. These designs provide more surface area for sedimentation while maintaining a compact size.

In conclusion, increasing the surface area of a sedimentation tank improves its efficiency by promoting better particle-fluid interaction, reducing settling time, and enhancing the clarification process. This allows for more effective separation of solids from the liquid stream, making it the correct answer to increase the efficiency of a sedimentation tank.