For this case we can translate the sentence given as:
"________ prefer watermelon and ____ prefer pears.
"Does ________ want apples?"
It is noted that the first part of the first sentence refers to the first person, so the pronoun to use is "Yo" (I).
The second part of the first sentence refers to the second person, so the pronoun to use is "Tú" (You).
Finally, the second sentence refers to a question asked to a third person in the singular, so the pronoun to use is "Ella" (She).
Thus, the phrase is completed as:
—Yo prefiero la sandía y tú prefieres las peras.
—¿Ella quiere manzanas?
Answer:
Yo; tú; Ella