……...is / are liable to saturation , respond to inhibitor and is under hormonal regulation and moves only along a concentration gradient .

Options

A. Simple diffusion
B. Facilitated transport
C. Active transport
D. Both facilitated and active transport

Answer

To solve the question, we need to identify the transport process that meets the following criteria:

1. **Liable to saturation**: This means that the transport process can reach a maximum rate when all available transport proteins are occupied.
2. **Respond to inhibitors**: The process can be affected by substances that inhibit the transport.
3. **Under hormonal regulation**: The transport can be regulated by hormones.
4. **Moves only along a concentration gradient**: The movement of substances occurs from an area of high concentration to an area of low concentration.

### Step-by-Step Solution:

**Step 1: Identify the Transport Mechanisms**
- There are three primary transport mechanisms: Simple Diffusion, Facilitated Transport, and Active Transport.

**Step 2: Analyze Simple Diffusion**
- Simple diffusion is a passive process where substances move directly through the membrane without the need for proteins or energy.
- It does not involve saturation or hormonal regulation, as it does not require specific proteins to facilitate the movement.
- Therefore, simple diffusion does not meet the criteria.

**Step 3: Analyze Active Transport**
- Active transport requires energy to move substances against their concentration gradient.
- It involves transport proteins and can be saturated, but it does not move along the concentration gradient; rather, it moves against it.
- Since the question specifies movement along a concentration gradient, active transport is not the answer.

**Step 4: Analyze Facilitated Transport**
- Facilitated transport is a passive process that requires specific membrane proteins to help move substances across the membrane.
- It moves substances from high to low concentration (along the concentration gradient), which meets the requirement.
- It can exhibit saturation when all available transport proteins are occupied.
- Additionally, facilitated transport can be regulated by hormones and can respond to inhibitors that affect the transport proteins.

**Conclusion:**
Based on the analysis, the correct answer is **Facilitated Transport**. This process is liable to saturation, responds to inhibitors, is under hormonal regulation, and moves substances along a concentration gradient.