In an uninformed search algorithm, what are "frontier" nodes?

Group of answer choices:

$1)$ Frontier nodes are nodes that are marked for pruning because they lead to loops or cycles, and revisiting them would result in inefficiency.

$2)$ Frontier nodes are the nodes that have been generated but have not yet been expanded, meaning their child nodes have not yet been explored.

$3)$ Frontier nodes refer to the nodes that are closest to the goal node, representing the boundary between explored and unexplored regions of the search space.

$4)$ Frontier nodes are nodes that have been fully explored, meaning all their child nodes have been generated and evaluated.