For this case we have the following statement:
I'm going to buy a new apartment ______________ I sell the old
one
We must find the correct word referring to the time in which the action will be performed.
The correct word that is related to time is:
as soon as
We have then:
I'm going to buy a new apartment as soon as I sell the old one
The translation of the sentence in Spanish is:
Voy a comprar un nuevo piso en cuanto venda el viejo
Answer:
B. en cuanto