Which of the following does not describe a rigid motion transformation?
a. translating a figure 2 units left
b. rotating a figure 90 degrees
c. reflecting a figure across the x-axis
d. dilating a figure by a scale factor of one half

I am not entirely sure, although as an educated guess, I would say D, dilating a figure by a scale factor of one half because you would be making the image smaller and 1/2 its size. 

D. Dilating a figure by a scale factor of one half

A rigid motion can can the location or rotation of a shape, but once it changes the size, it can no longer be considered one. Dilating by 1/2 would make it half as small, which therefore changes the size.

Hope I helped! :) (would you mind giving Brainliest? I'm trying to increase my title. I'd really appreciate it!)