"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.

[Gill714966]

I did the same, only because it was the first option, which is usually the safe one if I don’t want to have to repeat the item ad nauseam. No, not what I’m aiming for, which is to learn the language, only replicated/not replicated plurals seem to be randomly “right” in the absence of quantifiers so what to do?

[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.