The Indian Mathematician who gave the knowledge of the decimal system is:
A. Bhaskaracharya${\text{ - I}}$
B. Brahmagupta
C. Aryabhatta
D. Birbal Sahni

Hint: Scholars like Aryabhatta, Bhaskara \[{\text{II}}\], Varahamihira and Brahmagupta made important contributions to Indian mathematics. The decimal number system which we use today was first recorded in Indian mathematics. Many Indian mathematicians made important contributions to study the concept of negative numbers, zero as number, arithmetic and algebra.

Complete answer:
Aryabhatta was one of the greatest Indian mathematician astronomers who belonged to the Gupta dynasty. He discovered an approximation of the value of pi and he also correctly believed that the moon and the planets shine by reflected sunlight and the motion of the stars is due to the rotation of the Earth. He used zero in the decimal system.

He also formulated the first ten decimal places and gave algorithms to obtain square and cubic roots. The decimal system helped Indians to express large numbers without any efforts right from the Vedic age. This helped Indians to work with large time frames in the field of astronomy which helped them to obtain accurate results.

Therefore, option C is the correct answer.

Note: Aryabhatta also wrote a book called Aryabhatiya which was hugely influential.He was the first person to solve the Diophantine equations and he made major contributions to trigonometry and also algebra.