Q.In FPTP, winner is declared who: secures most postal ballots / belongs to highest-vote party nationally / has more votes than any other candidate in constituency / secures >50% votes?

FPTP does not require a majority of votes — it only requires beating every other individual candidate in that one constituency. …

Checking each option against how FPTP actually works:

• Secures the most postal ballots: postal ballots are only one small category of votes among many (in-person votes are the overwhelming majority); the winner is decided by total votes across all valid ballots, not postal ballots specifically.
• Belongs to the party with the highest number of votes nationally: FPTP winners are decided constituency by constituency; a candidate can win their own seat even if their party's national vote total is far behind another party's (as the 1984 election shows for several parties whose seats didn't match their national vote share). …