The correct answer is option 1: Indian Airlines. A natural monopoly occurs when a single firm can operate more efficiently than multiple firms due to economies of scale or barriers to entry. In the case of Indian Airlines, it can be considered a natural monopoly because of the high cost and complexity associated with operating an airline.

Option 2: Delhi Jal Board is not an example of a natural monopoly because there can be multiple providers of water services in an area, and it is not necessarily more efficient for a single entity to provide these services.

Option 3: Delhi Transport Corporation is also not an example of a natural monopoly as there can be multiple providers of public transportation in a city, and competition among these providers can lead to greater efficiency.

Option 4: Steel Authority of India is not an example of a natural monopoly either. While some industries may have economies of scale, there is usually active competition in the steel industry, making it difficult for a single entity to establish a monopoly.

Therefore, the correct answer is option 1: Indian Airlines, as it meets the criteria of a natural monopoly.