Introducing a woman, Shashi said, "She is the mother of the only daughter of my son." How that woman is related to Shashi ?

Let's decipher Shashi's statement layer by layer:
1. "My son's only daughter" $\to$ Shashi's granddaughter.
2. "The mother of the only daughter of my son" $\to$ The mother of Shashi's granddaughter, who is the wife of Shashi's son.
3. The wife of one's son is related as a daughter-in-law.

Hence, the woman is Shashi's daughter-in-law.