Someone has made an enquiry about my daughter, but she is not ready to marry yet. How do we answer without closing the door?

Say ‘not yet’ honestly, and let the man's side decide whether they want to wait. Do not give vague hopes that keep them waiting for years. For example: ‘Our daughter wishes to finish her studies first, about two more years. We do not want to tie anyone down, but if it is meant to be, our door is open.’ An answer like this respects their intention without creating an engagement by implication. Make sure your daughter herself decides how ready she is, not pressure from relatives. If the man is truly serious, he can come again at the agreed time. See signs you are ready to marry.