All whole numbers between 20 and 50 that have a difference of 4 between their digits

D. Acute.

Step-by-step explanation:

We are asked to find the type of triangle, if an ortho-center lies inside of a triangle.

We know that ortho-center of a triangle is the point, where all the three altitudes of a triangle intersect.

If the altitudes intersect inside a triangle, then triangle is an acute triangle.if the triangle is a right triangle, then the altitudes intersect at the the vertex opposite to hypotenuse . If the altitudes intersect outside the triangle, then triangle is an obtuse triangle.

Since we have been given that the ortho-center lies inside the triangle, so the altitudes will intersect inside the triangle. Therefore, the triangle must be an acute triangle and option D is the correct choice.