Yes, sandviş is correct too https://dexonline.ro/definitie/sandvi%C5%9F and more used also (sanviş is also used in spoken language but I don't know if it is accepted as a literary form).

Anyway, the other options for this exercise were incorrect for other reasons.

But it's a slang.

plural would be sandviciuri. this is irregular because it's a borrowed word. usually when the singular ends in i You pluralize by adding uri.