I would also pick the last one. Implicit means strongly and without doubt. Like my belief in God is implicit. It also means that a statement is not made directly but is implied meaning it is sure hinted at.
That's why I would pick the last one. It is implied that government by unwise people should not allow men who are imagination-free (meaning their imagination is free to think what it will), to be interrupted in the more important activity of just thinking.
It is a very convoluted sentence and not never is a difficult thing to interpret, but I think the idea of government by the unwise should be opposed.
