A is the answer because A us the only thing which we breath and the other 3 are what we eat.

Explanation:

Food:
Food refers to any nutritious substance that is consumed by living organisms in order to sustain life and provide energy, growth, and maintenance of body functions. It typically contains essential nutrients such as carbohydrates, proteins, fats, vitamins, and minerals.

a) Oxygen:
Oxygen is a colorless, odorless, and tasteless gas that is essential for the survival of living organisms. While it is necessary for respiration, oxygen is not classified as food. It is considered a gas and is inhaled by organisms to support the process of cellular respiration.

b) Mineral water:
Mineral water is a type of water that contains minerals and trace elements such as calcium, magnesium, and potassium. It is sourced from natural springs or wells and is often bottled for consumption. While it does not provide substantial calories or nutrients, mineral water is considered safe to drink and can contribute to hydration.

c) Soft drinks:
Soft drinks, also known as carbonated beverages or sodas, are flavored drinks that typically contain carbonated water, sweeteners, and various additives. While soft drinks may provide some hydration, they are generally high in sugar and calories, and are not considered a nutritious food choice.

d) Milk:
Milk is a nutrient-rich liquid that is produced by mammals, primarily cows, for the nourishment of their young. It contains a variety of essential nutrients such as proteins, carbohydrates, fats, vitamins, and minerals. Milk is consumed by humans as a beverage and is also used in the production of various dairy products such as cheese, yogurt, and butter.

Conclusion:
Among the options provided, oxygen is not classified as food. It is a gas necessary for respiration but does not provide nutrients or sustenance like food does.