Effective biocontrol option?

Which of the following examples might be an effective biocontrol option?

developing new herbicides to target specific groups of plants

reintroducing a large grazer, such as buffalo, that used to be dominant in an area, but has since been removed

introducing a generalist predator, such as cane toads, to control lots of different insects in an agricultural system, such as sugar cane fields.

introducing a highly-specialized insect herbivore, such as a weevil, that targets the seeds of a single invasive plant species, such as purple loosestrife.

introducing a pathogen, such as a fungi, that targets a whole group, such as grasses, because the group contains several invasive species.