In the group (G, *), where a, b ∈ G, which of the following is always true?

a + b ∈ G

ab ∈ G

a * b ∈ G

a - b ∈ G

In a group (G, *), the binary operation * is defined such that for all a, b ∈ G, a * b ∈ G.
This means that the result of combining any two elements of the group using the group operation is always an element of the group.