____, tu est plus beau que lui.
Translation:
[Blank], you are more beautiful than him.
Since this sentence is complete on its own and the question asks for the appropriate stressed pronoun, the blank must be filled with a repeat subject pronoun. The pronoun we have is "you," thus the blank will be filled with "you."
The options are:
A. Eux - "Them"
B. Lui - "Him"
C. Moi - "Me"
D. Toi - "You"
Remember, our subject pronoun is "you." A repeated stressed pronoun must agree with the subject.
Answer:
D. Toi
Toi, tu est plus beau que lui.