Declarer's weakest suit is diamonds in which he only holds an unprotected ♦K !
As long as North has the lead, Declarer remains safe, for he will be the last to play to a trick and his ♦K can not be captured.

But if South wins a trick he can then lead a ♦ through Declarer's ♦K, which will be captured if North holds the ♦A.

So which finesse must Declarer take in order to remain safe ?