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4th from your longest and strongest. Against 3NT, leading majors if possible is desirable. However, on this auction, you would be leading the opponents' suit. South has 4+ hearts. North has 4+ spades (otherwise he wouldn't have bothered with Stayman).

Rather than outguessing or outsmarting yourself, 4th from your longest and strongest against notrump is a tried and true method.

Partner started with QJ9.
If declarer had the J or Q, he would have won it; he wouldn't have wasted the A.

This is why 3rd hand plays cheapest of equals. It lets the person who led the suit know the layout.

If you answered #1, I suppose you could argue that it is technically correct, but it still got scored as incorrect. Sorry.

Take the K and play the 5. If you got this wrong, go back and review the previous question. You know partner has the QJ. Accordingly, you can set the contract. Play a low diamond (not the king to crash his honor) to him. He will take two diamonds and you will then have the setting tricks (A and K).
With touching honors, win the cheapest. Winning the A would deny the K.
As it turns out, declarer has only 8 top tricks. As long as you set up your diamonds in time, he can't set up his 9th trick.
See the remaining cards below. If you had ducked the Q, declarer would run for home with 9 tricks (1 spade, 1 diamond, 3 hearts, 4 clubs).

Thank you for attending our lesson on "Interpreting Partner's 3rd Hand Play."