The correct answer is option-2, Fructose.

Fructose is considered the sweetest natural sugar among the given options. It is a monosaccharide, commonly found in fruits, honey, and some vegetables. Fructose is approximately 1.7 times sweeter than table sugar (sucrose).

Option-1, Glucose, is also a monosaccharide and is found in various carbohydrates such as starch and sugars. However, it is not as sweet as fructose.

Option-3, Lactose, is a disaccharide present in milk and dairy products. It is not as sweet as fructose or sucrose.

Option-4, Sucrose, is a disaccharide commonly known as table sugar. It is made up of glucose and fructose molecules. Although it is sweet, it is not as sweet as fructose.

In conclusion, fructose is the sweetest natural sugar among the given options.