Answer: A.4 units
Explanation:
We know that if a figure is dilated by a scale factor of k , then the measure of the corresponding image (l') of a line segment having measure 'l' is given by :-

Given : Square RSTU dilates by a factor of 1\2 with respect to the origin to create square R'S'T'U'.
The measure of line segment R'S' = 2 units
The scale factor :

Using the above equation , we have

Hence, RS = 4 units.