Which of the following is an example of conditioned reflex ?

Options

A. Hand withdraws when pierced with a needle
B. Eyes close, when anything enters into them
C. During digestion, food goes forward in alimentary canal
D. Trained dog salivates when you ring a bell

Explanation

To determine which of the options is an example of a conditioned reflex, we need to understand the difference between conditioned and unconditioned reflexes.

### Step-by-Step Solution:

1. **Understand Reflexes**: Reflexes are automatic responses to stimuli. They can be classified into two types: conditioned reflexes and unconditioned reflexes.

2. **Identify Unconditioned Reflexes**: Unconditioned reflexes are natural responses that occur without prior learning. Examples include:
- Hand withdrawal when pierced with a needle (Option 1)
- Eyes closing when something enters them (Option 2)
- Digestion processes (Option 3)

3. **Identify Conditioned Reflexes**: Conditioned reflexes are learned responses that occur when a neutral stimulus is associated with an unconditioned stimulus. An example is:
- A trained dog salivating when a bell is rung (Option 4). This occurs because the dog has learned to associate the sound of the bell with being fed.

4. **Evaluate the Options**:
- Option 1: Hand withdraws when pierced with a needle - Unconditioned reflex.
- Option 2: Eyes close when anything enters them - Unconditioned reflex.
- Option 3: During digestion, food goes forward in the alimentary canal - Unconditioned reflex.
- Option 4: Trained dogs salivate when you ring a bell - Conditioned reflex.

5. **Conclusion**: The only example of a conditioned reflex from the options provided is Option 4: "Trained dogs salivate when you ring a bell."

### Final Answer:
The correct answer is **Option 4: Trained dogs salivate when you ring a bell.**

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