Final answer:
In the given sentence, 'thy' refers to 'thy father', indicating that the father of the person being spoken to is the subject of the sentence.
Step-by-step explanation:
The phrase 'The serpent that did sting thy father's life/Now wears his crown' is from a Shakespeare play and is indicative of the language and themes in classical literature.
In this passage, 'thy' is a second person possessive pronoun which refers to 'thy father', meaning the father of the person being addressed.
So, the correct answer to whom 'thy' refers to is option 2) Thy father.
This particular line suggests a betrayal and the usurping of power, symbolized by the serpent which is a common metaphor for deceit or evil.