you could use both "en que abra" or "en que abran" here in this sentence. If we use "en que abra" the subject would be "el nuevo centro comercial" (agreeing with the singular subject -> centro comercial / abra). But we used the plural "abran" referring to an impersonal "they" subject: "en que (ellos) abran el nuevo centro comercial". Here "ellos" refers to those in charge of the shopping centre, so the agreement is: ellos/ abran, and el nuevo centro comercial would be the direct object.
I hope this clarified it.
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