The correct answer is option 1: Nandas. During Alexander`s invasion of India, Magadha was ruled by the Nanda dynasty.

The Nandas were the rulers of Magadha from around 345 BCE to 321 BCE. They were of humble origins but managed to overthrow the ruling dynasty at the time, the Shishunaga dynasty, to establish their rule. The Nanda dynasty is known for its centralization of power and administrative reforms that helped in the expansion of the Magadha Empire. They had a strong military and established a vast empire that extended across northern India. However, the Nandas` rule was marked by cruelty and oppression, which contributed to their downfall. Shortly after the invasion of Alexander, the Nanda dynasty was overthrown by Chandragupta Maurya, who established the famous Maurya dynasty. Thus, while the Nandas were ruling Magadha during Alexander`s invasion, it was the Mauryas who came to power shortly after.