A relation between two variables a and b is a rule that takes some values a from a domain set and returns some value b in the range set.
The table says that, if you give 0, the relation returns 2. If you give 1, you get 6. If you give 4, you get -9. If you give 8, you get 8 back.
The domain is the set of values the relation can take, so here it is the set {0, 1, 4, 8}.
From the given choices, only 0 belongs to the domain.
