Swan neck flask experiment was conducted by Louis Pasteur in the mid-19th century to disprove the theory of spontaneous generation and support the theory of biogenesis. The experiment played a crucial role in the development of the field of microbiology and had significant implications for our understanding of the origin of life.

Spontaneous Generation and Biogenesis:
Spontaneous generation was a widely accepted theory in the past, which suggested that living organisms could arise spontaneously from non-living matter. For example, it was believed that maggots would spontaneously appear on decaying meat or that microorganisms would appear in broth. However, biogenesis proposed that living organisms can only arise from pre-existing living organisms.

The Swan Neck Flask Experiment:
To test the theory of spontaneous generation, Pasteur designed a simple yet brilliant experiment using swan neck flasks. He prepared two sets of flasks: one with straight necks and the other with swan necks. He then boiled broth in each flask to kill any existing microorganisms.

Key Findings:
1. Straight neck flasks: In the flasks with straight necks, the broth was exposed to the external environment, allowing dust particles and microorganisms to directly fall into the broth. In this setup, the broth quickly became cloudy, indicating the growth of microorganisms. This supported the theory of spontaneous generation.

2. Swan neck flasks: In the flasks with swan necks, the broth was boiled and the necks were bent into an S-shape. The shape of the neck prevented the direct entry of dust particles and microorganisms into the broth, while still allowing the exchange of gases. Despite being exposed to the same external environment as the straight neck flasks, the broth in the swan neck flasks remained clear and free from microorganism growth.

3. Breakage of the neck: In some of the swan neck flasks, Pasteur intentionally broke the necks after boiling the broth. This allowed the external air to directly enter the flask, resulting in the cloudiness of the broth due to the growth of microorganisms.

Conclusion:
The swan neck flask experiment demonstrated that the growth of microorganisms in broth was not due to spontaneous generation but rather due to the contamination from pre-existing microorganisms in the surrounding environment. The experiment provided strong evidence in support of the theory of biogenesis, which states that living organisms can only arise from pre-existing living organisms.

Therefore, the correct answer is option 'A' - the swan neck flask experiment proved biogenesis.