Explanation:

- Option 1: Louis Pasteur

- Correct Answer.

- In 1857, Louis Pasteur experimentally demonstrated that fermentation is a result of yeast and that airborne microorganisms can spoil fermentation by causing souring.

- Option 2: Martinus Willem Beijerinck

- Known for his work on the enrichment culture technique.

- He made significant contributions to microbiology but not directly related to yeast and fermentation in 1857.

- Option 3: Fanny Hesse

- She was known for introducing the use of agar in microbiological culture media.

- Her work does not relate to yeast or fermentation souring.

- Option 4: Alexander Fleming

- Discovered penicillin in 1928.

- His work was on antibiotics and bacterial growth, not fermentation.