Option A is the right answer as Ganga gives her son's name Devavrat. He was named Bhishma because he took a huge vow to mean bhishan pratigya.

King Shantanu and Ganga

King Shantanu was a great ruler and the son of King Pratipa. He was the king of Hastinapura, a kingdom in ancient India. Once while roaming near the banks of the river Ganga, he encountered a beautiful woman named Ganga. He instantly fell in love with her and expressed his desire to marry her. Ganga agreed to marry him but laid down a condition. She told Shantanu that he must never question her actions, no matter what she does. Shantanu agreed to this condition and they got married.

The Birth of the Son

After their marriage, Ganga gave birth to a son. However, right after the birth, she took the newborn baby and drowned him in the river Ganga. Shantanu was shocked and devastated by this act but he remembered his promise to Ganga and did not question her actions. This happened seven times as Ganga gave birth to seven sons and drowned each one of them.

Bhishma - The Eighth Son

When Ganga was about to drown their eighth child, Shantanu could no longer bear the pain and broke his promise. He questioned Ganga's actions and begged her to stop. Ganga then revealed that she was the goddess of the river Ganga and all her actions had a divine purpose. The sons she had drowned were actually Vasus, celestial beings who had been cursed to be born as humans. They had requested her to end their suffering by drowning them at birth.

Ganga agreed to Shantanu's request and spared the eighth child. This child was named Devavrata, but he is more commonly known as Bhishma. Bhishma grew up to be a great warrior and a noble prince. He was known for his unwavering loyalty and his vow to never marry or have children, which earned him the name Bhishma, meaning "the one who takes a terrible vow".

Thus, the name of the son given to King Shantanu by Ganga is Bhishma.