I'm not too sure of my solution here but here goes nothing. I picked (D) as both statements are sufficient.
We are asked to prove that whether the probability that the Participant picked is assigned to Reception Group is >5/8.
1) Statement 1: Selected Participants are not in the Materials Group is less than 45%
This statement can be interpreted as most of the members in the group are part of the materials group since the probability that they are not is less than 45%. And so if that is the case, then probability that they are either in P or R together is less than 45% - If there are a total of 100 people, then probability that they are either in P or R is 44 or less.
44% shows that the number of people in Reception Group - R is greater than 45% is not proved / made out. Hence Statement 1 is sufficient.
2) Statement 2: Probability that selected participant is not in Receptions Group is less than 40%
Which means that the probability that the selected participant is in receptions Group is more than 40%. As such 2/5 > 5/. Hence the probability that the selected participant is in reception group is answered as greater than 5/8.
As such, the Statement #2 is also sufficient.
Thus both statements are sufficient and answer is (D).
This is how I have done it. If it is wrong or not correct, I request some clarification kindly.