*Question E7A05:
Which of the following circuits continuously alternates between two states without an external clock signal?*  

*Correct Answer is  D: Astable multivibrator*  

### AI Response: (may contain errors)  

To understand why the correct answer is the astable multivibrator, let's break down the options:

### Correct Answer: Astable Multivibrator
- An **astable multivibrator** is a type of oscillator circuit that continuously switches between its two unstable states without needing an external clock signal. 
- It generates a square wave output, making it useful for applications like clock pulses in digital circuits.

### Why the Other Choices Are Incorrect:
1. **Monostable Multivibrator**:
   - This circuit has one stable state and one unstable state. It requires an external trigger to switch from the stable state to the unstable state and then returns to the stable state after a set time. Thus, it does not continuously alternate.

2. **J-K Flip-Flop**:
   - A J-K flip-flop is a bistable device that changes state based on its inputs (J and K) and a clock signal. It does not operate continuously without an external clock; it requires clock pulses to toggle its state.

3. **T Flip-Flop**:
   - Similar to the J-K flip-flop, the T flip-flop is also a bistable device that toggles its state with each clock pulse when the T input is high. It cannot alternate continuously without an external clock signal.

### Summary
- The **astable multivibrator** is the only circuit that continuously alternates between two states without needing an external clock, making it the correct answer.
- The other options rely on external triggers or clock signals to change states.

### Useful Rule of Thumb
- **Astable Multivibrators** are used for generating clock signals and square waves, while **monostable** and **bistable** circuits (like J-K and T flip-flops) are used for state storage and require external signals to change states.