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Pass. Not enough to enter the auction.

A. Leading a suit headed by ace-king is very desirable. In modern bridge, the correct card to lead is the ace. (In the old days, they taught to lead the king, but that made it too hard on partner to signal; he didn't know if you had AK or KQ). This ace-from-ace-king applies only on the opening lead. Later in the deal we revert to king from ace-king.

K. True, partner discouraged, but it isn't likely he had anything worthwhile to say.

It can't hurt to take your K; it won't set up anything useful for declarer (if he has Jxx the J won't do him any good).  There is no attractive reason to do anything else.

. From the fall of declarer's J, you can tell that declarer started with Jx and your 10 is good. But, playing that card would allow declarer to trump in dummy and throw a loser from his hand; the dreaded ruff-and-sluff. A real no-no.

There is no urgency to get aggressive here. From the bidding and your hand, you can tell partner doesn't have much. Declarer must have the missing aces. If partner has the K he'll get it--it can't go anywhere. If partner has the Q and you break that suit, it could be costly. Leading from the K is safe for now, but you might get thrown in with it later (more on that later).

Meanwhile a diamond is completely safe. You just want to get off lead and not give anything away. This is the equivalent to a punt in football. Just give the ball back to the other team.

Diamond or Spade. Don't feel bad if you prematurely clicked "diamond" or "spade"--you can take full credit even if you got scored as incorrect. :)
This is the remaining position:

We've shown a diamond play, but a spade is the same thing. The idea is to just exit safely and let declarer do his own dirty work.
As you can see, declarer will eventually have to break hearts and will lose a heart trick for down 1.
If you ever had played more clubs or broken hearts, the contract would have made.
This was the full deal:

 If you exited at trick 3 with a spade, that shouldn't work. Declarer will win, play the A next, then cash diamonds and endplay you with the K.

 For Lesson Deal #2 click here.