My initial thoughts:
Problem A: 3
Seems like the best description; it shows nearly half my hand in terms of suit, and my values. My other choice was the inverted raise promising at least one more diamond or something worse. I could trot out 1 NT, but with nothing higher than a nine, that begs disaster from my side.
Problem B: 2
Best description again. That diamond suit looks like six, and partner has only promised four hearts and three clubs if 4-4-2-3; the percentages suggest that partner has more clubs, but I would rather play in diamonds opposite a stiff or void than clubs opposite xxx.
Problem C: 3
This is close; if we were red, I would regard the try as automatic, but white at imps, pushing for game is not as important. With opener sitting behind me, I think it is worth the try for game.
Problem D: 3 NT The values are about right, the stop (with potential for a second stop) is right, the question is whether we have the tricks.
Problem E: 2
This is why I like playing weak NT; I bid my NT and follow partner's lead. On this auction, I have a decent suit to rebid, but tend to promise more length. Partner's negative double just forced me to rebid a five-bagger when a Moysian is not particularly good (especially in hearts where I would be ruffing with an honor).
Problem F: Pass This definitely feels like an underbid, but where am I headed. I need help for a notrump stopper. In spades, the wrong hand is tapped. If partner has three or four diamonds, that would be good, but difficult to get to except with a negative double and you are more likely to hear spades than diamonds. And clubs, you need partner to have at least five. If partner has a good hand, we will have another chance to bid; he is clearly short in hearts.
Problem G: 2 NT What I really want is 3
forcing for one-round, then if partner rebids hearts, I can bid notrump. 2NT and 3NT are both possibilities, but I really do not want to hear partner bidding 4
next.
Problem H:
J Trying dummy's second suit. It sounds like declarer has the club control, so maybe the king is well placed, and my trump length may give some problems, but I don't want to advertise. A heart looks wrong with LHO bidding the suit and I don't know if the top honors are together or split yet. That leaves diamonds, the sequence is also suggestive.