Explanation:

A median of a triangle is a line segment drawn from a vertex of the triangle to the midpoint of the opposite side. In other words, it connects a vertex to the midpoint of the side opposite to that vertex.

Triangles:
Triangles are 2-dimensional closed shapes that have three sides, three angles, and three vertices. The vertices are the points where the sides of the triangle intersect.

Median:
A median of a triangle is a line segment that connects a vertex of the triangle to the midpoint of the opposite side. Each triangle has three medians, one from each vertex.

Properties of Medians:
- The medians of a triangle intersect at a point called the centroid.
- The centroid is the center of mass of the triangle, and it divides each median into two segments. The length of the segment from the centroid to the vertex is twice the length of the segment from the centroid to the midpoint of the opposite side.
- The centroid divides each median into a 2:1 ratio. This means that the length of the segment from the centroid to the vertex is twice the length of the segment from the centroid to the midpoint of the opposite side.

Altitude:
An altitude of a triangle is a line segment drawn from a vertex of the triangle perpendicular to the opposite side. It is used to find the height of the triangle and is not related to the midpoint of the opposite side.

Conclusion:
In conclusion, the correct answer is option 'B' - median. A median connects a vertex of a triangle to the midpoint of the opposite side. This is one of the important properties of medians in triangles. An altitude, on the other hand, is a line segment drawn from a vertex perpendicular to the opposite side, and it is used to find the height of the triangle.