Among the options given, option 3 - Fish is considered the poorest source of fat. Fish is generally low in fat and contains healthy omega-3 fatty acids which are beneficial for the body. Omega-3 fatty acids have been shown to reduce the risk of heart disease and promote brain health. However, in terms of overall fat content, fish tends to have less fat compared to other animal-based sources such as curd (option 1), eggs (option 2), and milk (option 4).

Curd (option 1), eggs (option 2), and milk (option 4) are all relatively higher in fat content compared to fish. Curd and milk, being dairy products, contain varying amounts of fat, with full-fat versions containing the highest amount. Eggs also have a significant amount of fat, primarily found in the yolk.

Therefore, while fish is considered a healthy source of fats due to its omega-3 content, it is the poorest source of fat among the given options when considering overall fat content.