Ruling out the poorer leads in this nightmare choice, a trump gets zero because it may sacrifice a trick, especially since the opponents have not found a trump fit. A diamond lead, dummy's first-bid suit, is most unattractive, as well as dangerous. A club may find partner's strength at once—but it may be the suit in which declarer will have more losers if left to play it himself. That leaves spades, and the 9 gets a higher award because it is more deceptive. If declarer finds a possible finesse in the suit, he will probably refuse it, expecting the queen to be in your partner's hand.
Partner's slam double requests an unusual lead and probably shows a void, which is most likely, of course, to be in your long suit. The diamond lead, a poor second choice, just might set up a trick for the defense. The award for the spade lead? Oh, well, at least it isn't out of turn, and it's better than a trump lead, which is unthinking and unthinkable.
North's bidding has warned partner of probable club weakness; yet South has announced that he is prepared to play no trump against the expected lead of the unbid suit. The cards seem to be well placed for declarer, but you may infer that he is short in spades. A spade lead, therefore, is suggested, and the king receives the top award because it just might trap a singleton queen or jack in declarer's hand. The 10 of spades has the virtue of unblocking the suit. The orthodox 5 of clubs probably won't do much harm, but neither is it likely to be very effective.