1) Ф is a set containing no elements, at all.
2) { } is a set containing no members, at all.
3) If the solution has no certain answer, then it is considered as a null set or empty set.
4) {0} this is considered a null set because, we only use "count numbers" in problems involving sets
5) {A}=0 is another null set because, the set A is equivalent to "ZERO" (0), which is considered a non-counting number, so it excludes set A.

Hoped I helped! :))))