The hypothesis underlying team teaching is

Team teaching is an instructional arrangement in which two or more teachers jointly plan, teach, and evaluate a common group of students instead of each working alone with a single class.

The reform's founding hypothesis (associated with Judson T. Shaplin and the NASSP team-teaching studies) was resource-based: a school's most effective teachers are a scarce resource, so restructuring staff into teaching teams lets those best teachers reach - be shared by - a far larger number of students than the traditional one-teacher-one-class model allows.

Among the given statements, only the option about the best teachers being shared by more students restates this exact premise - using strong teachers more broadly, rather than merely describing a problem the reform happens to relieve.

• Feeling bored while working alone describes a possible side-effect on teacher morale, not the reform's founding rationale.

• Teachers not being competent is the opposite of the premise; team teaching assumes some teachers are exceptionally competent and should reach more students.

• A single teacher being unable to control the class is a classroom-management complaint, unrelated to the resource-sharing hypothesis.

So the hypothesis underlying team teaching is that a school's best teachers should be shared across more students, rather than being confined to one class alone.