The best paraphrase of Benvolio's lines is Once you see other beautiful girls at the party, you will see that Rosaline is not so special.
We can see all the keywords in the lines:
At this same ancient feast of Capulet’s (fest is a synonym of party),
With all the admired beauties of Verona: Go thither; and, with unattainted eye Compare her face with some that I shall show, And I will make thee think thy swan a crow. (this part compares Rosaline with all the girls at the party and he assures him that he could surely find a so much more beautiful girls there)
The other options are not correct because:
If you go to the party you will see that Rosaline is, in fact, the most beautiful girl in Verona. (this sentence expresses the complete opposite of Benvolio's message)
Compare Rosaline with other girls from Verona and then decide whether she is worthy of your love. (It is not about deserving love or not, it is about Romeo realizing that she is not special at all)
There are many girls in Verona who are just as beautiful and desirable as Rosaline. (This sentence only talks about half of Benvolio's idea, reason why it is incomplete)
