Final answer:
The correct phrase to complete the sentence is "en cuanto." It means "as soon as" or "when."
Step-by-step explanation:
The correct phrase to complete the sentence is en cuanto. The sentence translates to "I am going to buy a new apartment as soon as I sell the old one." The phrase en cuanto means "as soon as" or "when." It is used to indicate that the action of buying a new apartment will happen immediately after selling the old one.
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