Final answer:
The correct demonstrative adjective to fill in the blank in the given sentence is 'aquel'. 'Aquel' is used to refer to something that is further away from both the speaker and the listener.
Step-by-step explanation:
The correct demonstrative adjective to fill in the blank in the given sentence would be aquel. The sentence is comparing the attractiveness of two boys, and the use of the demonstrative adjective 'aquel' indicates that the second boy being described is further away in distance or time from the speaker compared to the first boy mentioned. 'Aquel' is used to refer to something that is further away from both the speaker and the listener. Here's an example:
Este sombrero es bonito, pero aquel sombrero es más bonito.
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