Why of the following is called fruit sugar? 

Fructose 

The correct answer is option 4. Fructose.

Fructose is sometimes referred to as "fruit sugar" since it is largely found naturally in many fruits. Other plant foods including honey, sugar beets, sugar cane, and vegetables also naturally contain it. The sweetest naturally occurring carbohydrate is fructose, which is between 1.2 and 1.8 times sweeter than sucrose (table sugar).