what is minimal pairs

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Minimal pairs are two words that differ in a single phonological element and have distinct meanings. Tacks/tax and mail/male are pronounced the same, despite having distinct meaning.
Thus the tacks/tax and mail/male examples would actually be considered homophones, not minimal pairs, I think.

To add to CBauserman's answer:

You would need to make two changes, not one, to change place into face, so they aren't a minimal pair.

Thus, of the ones you listed, only man - pan and flat - flap are minimal pairs.