"These are formal clothes."

Translation: Ini pakaian resmi.

[Tannuska]

I wrote Ini pakaian-pakaian resmi. pakaian-pakaian = plural was the first option in the possible answers. Then it should be accepted.

[Tom307173]

Would ini adalah pakaian resmi also be correct?

[Rick392366]

Would ini adalah pakaian resmi also be correct?

Yes, that is also correct.
Using "adalah" is the more formal way to say it.