Answer:
A⊆B
Step-by-step explanation:
C⊆D if the set C contains only elements from set D.
Let's look at the first choice:
B⊆A is not true because 3 and 4 are not in A.
Second choice:
3⊆A is not true because 3 is not a set; it is an element.
Also, 3 is not even in {1,2}.
Third choice:
See explanation for choice 2.
Fourth choice:
A⊆B is true because all the elements in A are contained in B.
That is the elements 1 and 2 are contained in the set {1,2,3,4} which is B.
