Which best explains why the orthocenter of an obtuse triangle is outside the triangle?

All three of the altitudes lie entirely outside the triangle.

Step-by-step explanation:

The orthocenter is the center of the triangle formed by creating all the altitudes of each side.  

The altitude of a triangle is formed by creating a line from each vertex that is perpendicular to the opposite side.  

In acute traingle , the orthocenter lies inside it.

In right angled triangle, the orthocenter lies on the triangle.

In obtuse triangle , the orthocenter lies outside the triangle because all the three altitudes meet outside .

So, the best explains why the orthocenter of an obtuse triangle is outside the triangle : All three of the altitudes lie entirely outside the triangle.