If a tendered offer is made for 15.00 and the stock closes at 15.50 on the closing date of the tender, what will the price paid for the tender be? 15 or 15.50?

$15, per the terms of the tender. You have basically sold your sahres to the company that made the tender offer at the agreed-upon price of $15. The only way the stock would close above the tender price on the final day of the offer is if the market feels that the buying company has failed to buy a majority of the shares.

The fact that the company will be delisted after the tender date, those who do not tender are left with worthless paper.