Question

Form the union for the following sets.

Answer 1

Union of two sets R and S is the set which contains all element, which are in either set R or S. The symbol for union is ∪, so the union of two sets is written as R∪S. 
In our case, the sets are:
R = {10, 15, 20}
S = {20, 25}
The union for the sets R and S are: 
D. {10, 15, 20, 25}
Intersection on the other hand is the set that contains the elements that belong both in set R and set S. 

Answer 2

It's D.) {10,15,20,25}