Option 1: The pictures were small in size.

This option suggests that the reason Mughal art was known as Miniature Paintings is because the pictures themselves were small in size. However, this is not entirely accurate. While it is true that miniature paintings are generally small in size, this is not the sole reason for their categorization as such. Miniature paintings are characterized by their intricate and detailed work, regardless of their size.

Option 2: The pictures covered small themes.

This option suggests that miniature paintings were called so because they covered small themes. This is incorrect. Miniature paintings can depict a wide range of themes, from historical events to religious stories to courtly life. The small size of the paintings does not limit the subjects they can portray.

Option 3: The painting used small canvas.

This option implies that miniature paintings were called as such because they were created on small canvases. While miniature paintings are typically small, they are not limited to being painted on canvas. They can also be found on other surfaces such as paper, ivory, and wood.

Option 4: They were mainly the small-sized reproductions of the normal-sized originals.

This is the correct option. Miniature paintings were often small-sized reproductions of larger paintings or manuscripts