Final answer:
Louis Pasteur's research established that alcohol fermentation is caused by microorganisms known as facultative anaerobes, which can operate in both aerobic and anaerobic conditions.
Step-by-step explanation:
French scientist Louis Pasteur, through his research in the mid-19th century, established that fermentation to produce alcohol is caused by microorganisms. In his experiments, Pasteur demonstrated that the fermentation process is biological, not purely chemical, and is initiated by the action of living yeast cells. These cells, called microbial fermentation agents, can function in the absence of oxygen, which is known as an anaerobic process.
Many of these microorganisms are known as facultative anaerobes, which can switch between aerobic respiration (requiring oxygen) and fermentation (no oxygen required), depending on the presence of oxygen. Pasteur's research laid the foundation for understanding how microorganisms can cause both fermentation and disease, influencing the fields of microbiology and food preservation. His discoveries showed that, unlike obligate anaerobes which cannot survive in the presence of oxygen, facultative anaerobes use fermentation adaptively when oxygen levels are low.
Given the options provided in the question and the information that fermentation to produce alcohol can be carried out by prokaryotes, especially those that can function both in the presence and absence of oxygen, the correct answer is A) facultative anaerobes.