In Pavlov's experiments, which of the following was used as a stimulus for development of an acquired reflex?

Options

A. Food
B. Smell of food
C. Sound of bell
D. Both (1) and (2)

Answer

To solve the question about Pavlov's experiments and the stimulus used for the development of an acquired reflex, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Conditioned Reflex**:
- A conditioned reflex, also known as an acquired reflex, is a learned response to a previously neutral stimulus that has been paired with an unconditioned stimulus.

2. **Identify the Experiment**:
- Ivan Pavlov conducted experiments in the early 1900s using dogs to study their salivation response.

3. **Recognize the Unconditioned Stimulus**:
- In Pavlov's experiments, the unconditioned stimulus was food, which naturally elicited salivation from the dogs.

4. **Identify the Conditioned Stimulus**:
- Pavlov introduced a neutral stimulus (the sound of a bell) before presenting the food. Over time, the dogs learned to associate the sound of the bell with the presentation of food.

5. **Observe the Response**:
- Eventually, the sound of the bell alone was enough to trigger salivation in the dogs, demonstrating the development of a conditioned reflex.

6. **Conclude the Correct Answer**:
- The stimulus used for the development of the acquired reflex in Pavlov's experiment was the sound of the bell.

### Final Answer:
The correct option is the **sound of the bell**.