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Spade or diamond.

Definitely not the A or underleading it (a no-no against suit contracts).
A trump lead on this auction would be passive and acceptable from, say, xxx. But a lead from the jack is too dangerous and could cost a trump trick.

It isn't like a diamond or spade is wonderful, but it is the least of evils.
A spade looks a little safer than a diamond, so let's go with the 2.

Declarer has the K and the 9. If declarer had the J, he would have won it cheaply (instead of taking the king). So, our partner has the J10. Our partner won't have the 9, or he would have played it (cheaper of touching). So, declarer has the 9.

6. It looks like partner has the A. You want to try to set up a second spade trick before declarer can use dummy's diamonds.

From trick one, you know partner has the J, so it is safe to play spades. This turns out to be the killing defense, as the full deal is below:

The defense gets a hard-earned spade trick along with ace-king-ace in the minors for down one.

Click here to play Lesson Deal #4.