Fermentation and Alcohol Levels:
Fermentation is a natural process in which yeast converts sugar into alcohol and carbon dioxide. It is the initial step in the production of alcoholic beverages such as wine and beer. During fermentation, yeast consumes the sugars present in the ingredients and produces alcohol until the alcohol concentration reaches a certain level. At this point, the alcohol inhibits further fermentation, and the process stops naturally. This is why the correct answer is option 'A' - fermentation is inhibited at an alcohol level of 10-18%.

Alcohol Tolerance of Yeast:
Yeast, the microorganism responsible for fermentation, has a limited tolerance for alcohol. Different strains of yeast have varying levels of alcohol tolerance, but generally, most yeast strains start to struggle to survive when the alcohol concentration in the fermentation mixture exceeds 10-18%.

Effects of High Alcohol Levels:
When the alcohol concentration becomes too high, it creates a toxic environment for the yeast. The alcohol molecules start to interfere with the yeast's metabolic processes, inhibiting its ability to consume sugars and produce more alcohol. This leads to a slowdown or complete halt in the fermentation process.

Distillation and Alcohol Concentration:
Distillation is a process that involves heating the fermented liquid and collecting the evaporated alcohol, leaving behind most of the water and other impurities. This technique allows for the concentration of alcohol to be increased beyond the limits of fermentation. By separating the alcohol from the fermented mixture, distillation enables the production of stronger alcoholic beverages such as brandy and whisky.

Purification and Quality Improvement:
Distillation not only increases the alcohol concentration but also helps purify the beverage. During the distillation process, impurities, including certain compounds that contribute to off-flavors, are left behind, resulting in a cleaner and more refined product. This purification step contributes to the improved quality and flavor profile of brandy and whisky.

Conclusion:
In summary, fermentation alone can only produce alcoholic beverages up to a certain alcohol level due to the limited alcohol tolerance of yeast. Distillation is necessary for beverages like brandy and whisky as it allows for the concentration of alcohol beyond what fermentation alone can achieve. Furthermore, distillation purifies the beverage and improves its quality by removing impurities, resulting in a more refined and enjoyable drinking experience.