Good question. The rule about D is that exactly one other event occurs with it. That means that if D is selected, exactly two are selected.
To make sure that exactly two are selected, we need to make sure that none of the items selected with D force other items to be selected.
This would leave us with several options, but the problem is the second rule about B. If B is not selected then at least one item (H or I) must be selected.
In other words, the selection will always include at least
B or H or I.
Since there is only one other space available if D is selected, that space must be occupied by one of these three items. No other item is possible.
Therefore, we start with these three possibilities, but look what happens with H and I. H forces E and F to be selected as well. This doesn't work. I forces K to be selected. This doesn't work.
Therefore, the only possibility is B.
I hope this helps!