Ans 1:

Superset:

A set A is subset of a set B if A is contained inside B i.e. every element of A is an element of B.

if A is a subset of B, then B contains A or B is the superset of A.

If B is the superset of A, then it is written as B ⊃ A

Ex: The set {a, b, c, d} is the superset of the set {b, c, d}

Universal Set:

A set contains all sets in a given context is called the universal set. It is the superset of each of the given set.

Generally, it is denoted by U.

Ex: Let A = {1,2,3}, B = {2,4,5,6} and C = {1,3,6,7}

Then Universal set U can be taken as

U = {1,2,3,4,5,6,7} because it contains all elements from the given sets.

Ans 2:

Given A = {1,2,3} and B = {1,2,3,4}, then B is superset of A.

Yes, we can say that B is the recommended universal set because it contains all elements and no other set is given.

So U = B = {1,2,3,4}