The correct answer is option 'A' - Astable multivibrator.

Explanation:
A multivibrator is an electronic circuit that generates continuous square wave pulses. It consists of two amplifying stages connected in a feedback loop, which allows the circuit to oscillate between two stable states.

There are three types of multivibrators: astable, bistable, and monostable. Each type has different characteristics and applications. In the context of a clock timer, the astable multivibrator is the most suitable choice. Here's why:

1. Astable Multivibrator:
An astable multivibrator is a type of multivibrator that does not have a stable state. It continuously oscillates between two unstable states, producing a square wave output. It is also known as a free-running multivibrator or a relaxation oscillator.

2. Clock Timer Function:
A clock timer is a device that generates regular pulses at a fixed frequency to measure time or synchronize operations. It is commonly used in digital systems, microcontrollers, and various electronic applications.

3. Astable Multivibrator as a Clock Timer:
The astable multivibrator is widely used as a clock timer because of its continuous oscillation and ability to generate regular pulses. Its output waveform has a fixed frequency and duty cycle, making it suitable for timing applications.

4. Frequency and Duty Cycle Control:
In an astable multivibrator, the frequency and duty cycle of the output pulses can be controlled by varying the values of resistors and capacitors in the circuit. This allows the clock timer to be adjusted according to the desired timing requirements.

5. Stable Frequency:
Unlike the bistable multivibrator, which requires external triggering to change its state, the astable multivibrator generates continuous oscillations without any external input. This ensures a stable and continuous clock signal.

In conclusion, the astable multivibrator is the preferred choice for a clock timer due to its continuous oscillation, ability to generate regular pulses, and frequency control. It can be easily implemented using basic electronic components and is widely used in various electronic applications.