My suggestions: The key to the main doors has been lost; The dishes for use at official dinners have to be washed . Avoiding the complete relative clause often makes the sentence more easily read. However, both a.
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Doctor DA is correct, although you need commas. C is awkwardly said and should be reworded. B and D are not correct at all.A needs commas if you consider the relative clause non- defining. If you consider the relative clause defining, no commas are required.