Option (c) : padma nand.

But I think there is a mistake, the biggest enemy of chandragupta is "Dhana nand".

Chandragupta Maurya was an ancient Indian emperor who founded the Maurya Empire in the 4th century BCE. He is known for his military conquests and the establishment of a centralized and well-organized administration. One of his biggest enemies was Dhana Nanda, also known as Dhana Nanda or Dhanananda.

Dhana Nanda: The Biggest Enemy of Chandragupta
Dhana Nanda was the last ruler of the Nanda Dynasty, which was ruling over Magadha (present-day Bihar) at that time. He is often considered as one of the most despotic and arrogant rulers in Indian history. Dhana Nanda's oppressive rule and his excessive taxation policies made him highly unpopular among his subjects.

Chandragupta's Rebellion Against Dhana Nanda
Chandragupta, who was born into a humble family, grew up witnessing the atrocities of Dhana Nanda's regime. He decided to gather support and rebel against the oppressive ruler. In his pursuit of overthrowing Dhana Nanda, Chandragupta formed an alliance with various regional kings, including Parvataka, the King of Takshashila (present-day Pakistan) and Seleucus I Nicator, the Macedonian general of Alexander the Great.

The Battle of Pataliputra
Chandragupta's forces, combined with the support of his allies, marched towards the capital city of Pataliputra (present-day Patna). The battle between Chandragupta and Dhana Nanda's armies was fierce, with both sides displaying their military prowess. However, Chandragupta's strategies and the support of his allies proved to be decisive, and he emerged victorious.

Establishment of the Maurya Empire
After defeating Dhana Nanda, Chandragupta ascended to the throne and established the Maurya Empire, which would become one of the largest and most powerful empires in ancient Indian history. Chandragupta's victory over Dhana Nanda marked the end of the Nanda Dynasty and the beginning of the Maurya Dynasty.

Importance of Defeating Dhana Nanda
The defeat of Dhana Nanda was a significant turning point in Chandragupta's life and the history of ancient India. It not only paved the way for the establishment of the Maurya Empire but also liberated the people from the oppressive rule of Dhana Nanda. Chandragupta's victory brought about a period of political stability and economic prosperity in the region.

In conclusion, Dhana Nanda was the biggest enemy of Chandragupta Maurya. The battle between them resulted in Chandragupta's victory and the establishment of the Maurya Empire. This event marked the end of the Nanda Dynasty and brought about a new era of political and economic prosperity in ancient India.