It’s not necessary to the argument that reducing class size be extremely expensive. It might help, but it isn’t necessary. The argument needs something that compares the costs and benefits of reducing class sizes versus recruiting and retaining teachers.

This provides the basis to conclude that spending money on recruitment and retention is a comparatively better use of funds than reducing class sizes. The stimulus only talks about the benefits on an absolute basis — this answer choice compares the two options.

It doesn’t matter how easy it is to get approval — it only matters whether it is the best use of funds given the available options.

This answer choice doesn’t make sense in the context of the argument. The author argues that efforts to recruit and retain teachers are a better use of funds than reducing class size. But if one leads to the other, then we don’t need to choose between the two.

This doesn’t make sense in the context of the argument. The argument is about determining what use the funds should be spent on. If it is difficult to identify which would be most productive, this certainly doesn’t support the conclusion that one use is better than the other.