Which of the following represents a radius of the circle?
a.ts
b.sn
c.tr
d.mn

MN because it is half. The way i always remember the difference between radius and diameter is because i once saw a picture of a woman and a small boy standing against a fallen tree stump. the kid came up half way and the mother was as tall as the log. The mothers name was diameter and the kid was radius, you could try to remember it this way it would make it easier.  

Is there a picture of the triangle in the question?