the answer Box A - Bounty - 8 kg
Box B - Dairy Milk - 3 kg
Box C - Kit-Kat 11 kg
Box D - Mars - 5 kg
It is given that the box with Kit-Kat is between the boxes containing 3 kg and 5 kg chocolates. There are two possibilities:
[(3), Kit-Kat, (5)] or [(5), Kit-Kat, (3)]
Also, the Dairy Milk box is between Bounty and Kit-Kat box. Now, there are four possibilities:
[Bounty, Dairy Milk (3), Kit-Kat, 5] or [3, Kit-Kat, Dairy Milk (5), Bounty] or
[Bounty, Dairy Milk (5), Kit-Kat, 3] or [5, Kit-Kat, Dairy Milk (3), Bounty]
It is obvious that the remaining box contains Mars, which weighs either 3 kg or 5 kg. Also, the weight difference between Mars and Bounty is 3 kg. It means that Bounty is 6 kg, 2 kg or 8 kg. Hence, Kit-Kat is 11 kg.
[Bounty, Dairy Milk (3), Kit-Kat (11), Mars (5)] or [Mars (3), Kit-Kat (11), Dairy Milk (5), Bounty] or
[Bounty, Dairy Milk (5), Kit-Kat (11), Mars (3)] or [Mars (5), Kit-Kat (11), Dairy Milk (3), Bounty]
As Box A and D do not have the least amount of chocolates in it, Bounty can not be 2 kg. Also, Box A and Box D can not weigh 3 kg. Thus, the only possibility is:
[Bounty (8), Dairy Milk (3), Kit-Kat (11), Mars (5)]