The most specific thesis statement is the first one: Romeo and Juliet explores how a family feud can result in tragedy for young lovers, since it tells us what happens with the plot of the play. Options 3 and 4 are not only vague, they have little to do with the plot of the play (while Juliet's youth is commented upon, it is not a major plot point, and while the Friar is a religious figure, there is no foregrounding of religion as a factor in Romeo and Juliet's love story). Option 2 is too vague; it tells us about a "famous love story" without any specific details, unlike the first statement, which is the correct one.
