In the figure below which term best describes point X?

The answer is B. centroid, which is the intersection of three medians of the triangle's sides.
Let's look into other answers as well:
A. Circumcenter
It is the point where three perpendicular bisector meet, and is not correct as we can see there is no indication of right angles.
The first photo is what a circumcenter looks like:
C. Incenter
It is the point where the angle bisectors of the triangles meet, which is not indicated in the photo as well.
The second photo is what a incenter looks like:
D. Orthocenter
It is a point where the altitudes of a triangle intersect, and is not indicated by the figure.
The second photo is what a Orthocenter looks like:
Therefore the answer is B. Centroid.
Hope it helps!