The sign C here refers to subset.
A subset is a set in which all its elements are elements of the universal set.
This sign above represents element. It is put in between an element and a set to indicate that the element (on the left) is a member of the set.
A) This is FALSE because all the elements of the set on the left are not found on the set on the right.
B) This is TRUE because all the elements of the set on the left agree with those on the right (order is not considered)
C) This is TRUE because 10 is an element of the set. The ... following 5 indicates that the elements go on.
D) This is FALSE because (Joe) is subset and not an element as is put. However, "Joe" without the braces will be an element.