Could u please say what does 'd' mean I mean that value is for number of what

Whenever we draw venn diagrams, it is important to note that there is always a universal set in which the entirety of daughter sets are contained. So, here d refers to the number of elements which are present in no other set as a result of which they refer to the leftovers.

As an example, consider a set of 10 people. Let's say 2 of them like coffee, 1 likes tea, and 3 like cold drink. Then in this case, d= 10-(2+1+3)= 4.