Juliet struggles with her grief over her cousin’s death at the hands of her husband. - [Person versus self]
Here it is very clear that the struggle is happening inside Juliet's head. She is not in a conflict with someone else, nor with the unknown, since the reason of her conflict is her grief and her feelings for her husband.
The hatred between the Montagues and Capulets intensifies as the action progresses. [Person vs Person]
Capulets and Montagues are people from different family. This one is very much self explanatory.
Friar Laurence’s letter does not reach Romeo due to an unforeseen accident. [Person vs Unknown]
Since we don't know what type of accident it was and what involves it at all, we can only say that a person (Friar Laurence) it put against the unknown.
