The correct answer is D.
Teachers should study educational philosophy because it can help them to clarify their own philosophy of education, which is the foundation of their teaching practice. By understanding the different philosophical perspectives on education, teachers can develop a more informed and coherent approach to their work.
Option A is incorrect because it assumes that teachers do not know educational philosophy. This is not necessarily the case; many teachers have a strong understanding of educational philosophy, and some may even have a degree in the subject.
Option B is incorrect because it assumes that teachers do not have their own philosophy of education. This is also not necessarily the case; many teachers have a clear idea of what they believe about education, even if they have not formally studied educational philosophy.
Option C is incorrect because it assumes that philosophy is the backbone of all disciplines. This is not a universally accepted view; some people believe that philosophy is a separate discipline from other fields of study.
In conclusion, the correct answer is D. Teachers should study educational philosophy because it can help them to clarify their own philosophy of education, which is the foundation of their teaching practice.