The underlying decision revolves around trying to create a defensive trick either for yourself or partner.
(a) Do NOT cover. If declarer has J10x covering will create 2 tricks for declarer. It can’t gain to cover.
(b) DO cover. You are trying to create a trick for partner holding 10xx. If you fail to cover, declarer will finesse twice for 3 tricks. If declarer holds J10x it doesn’t matter what you do.
(c) Do NOT cover. Declarer will have to play ♦ A on the second round of the suit and your ♦ K is still intact. Covering wouldn’t have been smart if declarer holds QJ10xx
(d) DO cover. You are destined to make 2 tricks with your ♣ 109. If declarer has ♣ QJ then it wasn’t essential to cover ♣ Q, but you are going to feel silly if declarer led a sneaky ♣ Q from ♣ Qx and partner holds ♣ Jxx.
As a general rule
With one high honour in dummy, cover the SECOND honour led.
With two high honours in dummy, cover the FIRST honour led.
THE OBJECTIVE IS TO CREATE A TRICK OR TRICKS FOR YOU OR YOUR PARTNER SO DON’T FOLLOW RULES BLINDLY