These are words are relative pronouns, but can they ever be used in a question as an interrogative pronoun? I assume that function is reserved for Cuál and Cuáles without el-lo-la-las.
Next, could "cuáles" be used in this quiz sentence: Voy a preguntar a la dependienta ________ lavadoras son más eficientes. trying to ask: which ones of these washing machines are more efficient?
You assumed correctly, for the interrogative ones you'd use "cuál, cuáles" without the articles.
For your sentence "Voy a preguntar a la dependienta ________ lavadoras son más eficientes." you can indeed use "cuáles lavadoras..." but that is mainly used in Latin America, not in Spain. In Spain you'd use "qué lavadoras...".
I hope this clarified it.
Gracias y saludos
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