If r, s, and t are collinear and rs + st = rt, which of the following is true? a. t is between r and s.
b. rs=rt
c. s is the midpoint of.
d. s is between r and t.

The answer is D. S is between R and T.

Imagine a line, which a starting point is R and a final point is T. And somewhere in between a point S. 
The total distance is RT (from the starting to the final point).
The distance RS is one part of the line, and the distance ST is the other part of the line. If these two distances sum up, you will get the total distance:
RS + ST = RT