Questions

It is ABACUS.

Words formed are as below 

BUS
CAB
CUB
US

Baburnama was the autobiography of founder of the Mughal Empire Babur. It is wrtten in Chagatai Turkic language, which is his mother tongue.

Environmental science is a field that integrates physical, biological and information sciences to the study of the environment, and the solution of environmental problems. This emerged from the fields of natural history and medicine during the Enlightenment.

In brief, it is a scientific study of the natural world and how it is influenced by people.

Mihirakula, also Mahiragula, was one of the most important rulers of the Alchon Huns, who led a conquest and gained temporary control of Gandhara, Kashmir, northern and central India. Mihirakula was a son of Toramana, both of Huna heritage, and ruled the Indian part of the Hephthalite Empire.