"La secadora está disponible en negro."
Translation:The dryer is available in black.
Castellano, much like English, is going to work this way. In English, if someone said Dyer is available in black - your mind would go to clothes dryer NOT hair dryer. If in English I meant hair dryer, I'd have said Hair dryer, not just Dryer. Now you can squawk all you want that I could have meant either, and technically you'd be correct, however at the same time, it would be a silly thing to argue. Same with this sentence. Since it didn't say "secadora de pello, or secadora de cabello", then one just would assume it's a clothes dyer.
You can plead your case to Duo, but I'd have to wonder why you'd bother?
Webster's New World Dictionary fourth edition defines 1.drier (noun) as "a substance added to paint, etc. to make it dry faster" and 2. DRYER - Adj as in this shirt is dryer than that shirt. DRYER is defined as a noun: One that dries; specif. an appliance for drying clothes with heat.
Therefore 'The dryer is available in black' should be the only correct answer according to Webster.