Final answer:
The line that foreshadows Romeo's fate in Act I of Shakespeare's 'Romeo and Juliet' is Tybalt's pronouncement of his intent to kill Romeo without considering it a sin.
Step-by-step explanation:
In the excerpt from Act I of Shakespeare's Romeo and Juliet, the line that foreshadows Romeo’s fate is 'Now, by the stock and honor of my kin, To strike him dead, I hold it not a sin'. This line represents Tybalt’s intense hatred for the Montagues, a sentiment that ultimately contributes to the tragic outcome of the play. The animosity Tybalt displays here foreshadows the violent encounters to come, including Romeo's own tragic end.