noun
DUBIOUS : After making several inquiries I finally discovered his address.
GOOD : After making several enquiries, I finally discovered his address.
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In a simple request for information, the usual word is enquiry : 'We've had a lot of enquiries in response to the advertisement.'
When you mean 'a full investigation or a long serious study', the usual word is inquiry : 'a court of inquiry', 'There is to be an official inquiry into the cause of the crash.'