Total no. of cards = 7
These 7 cards are randomly put into 3 different boxes.
Box 1 = gets 4 cards
The number of ways to choose 4 cards out of the 7 cards for box 1 = 7C4
Hence, box 1 gets 4 cards remaining card = 3
Box 2 = gets 2 cards
The number of ways to choose 2 cards from the remaining 3 cards = 3C2
Box 3 = gets the remaining 4
The number of ways to choose 1 card out of the remaining 1 = 1
Total no. of sets that can be created = (7C4) * (3C2) * 1
$$=\left[\frac{7!}{\left(7-4\right)!\cdot4!}\cdot\frac{3!}{\left(3-2\right)!\cdot2!}\right]\cdot1$$
$$=\left[\frac{7\cdot6\cdot5\cdot4!}{3\cdot2\cdot1\cdot4!}\right]\cdot\left[\frac{3\cdot2!}{1\cdot2!}\right]\cdot1$$
$$=\left[7\cdot5\right]\cdot\left[3\right]\cdot1=35\cdot3\cdot1=105$$
Answer = option B