Question

If set D is not the empty set but is a subset of set E, then which of the following is true?

Answer 1

Concept:
If every element of a set D is an element of set E, then D is the subset of E.
e.g
D={1,2,3},   E={1,2,3,4}
As all the elements of D are in E, so D is the subset of E.
Now the intersection of two sets D and E is the set of all elements which belong to both D and E.
As, in our example the set D={1,2,3} is the intersection of set D and set E.

From the above discussion, it is proved that If set D is not the empty set but is a subset of set E then 
 D ∩ E = D is the true answer.