In the Subset Symbol?

The symbol "⊆" means "is a subset of". The symbol "⊂" means "is a proper subset of". Since all of the members of set A are members of set D, A is a subset of D.

What is the difference between ∈ and ⊆?

If you use the ∈ mark, that is only for one element. If you use the ⊆ mark, that is for a set.

What does ∪ mean in math?

The union of a set A with a B is the set of elements that are in either set A or B. The union is denoted as A∪B.