Answer:
R indicates Random Assignment
Explanation:
In designing an experiment, R is the symbol used for assigning the subjects randomly to different groups through randomization. It is called random assignment and It is the option (d) random assignment. It is achieved by using chance procedure ( random number generator, by flipping coin, throwing ). This ensures that all the subjects have equal chance of placing in all the groups.
Option (a) Symbol O is used for Observations. (So option (a) is wrong answer)
Option (b) Symbols x , y are used as Observational variables (So option (b) is wrong answer)
Option (c) Comparison groups is wrong answer as R is not used for that.