Correct answer is A. I understand that my original answer was wrong because the reinforcement for group 2 was provided for every 3 times the lever is pulled, but isn't this an example of a fixed reinforcement schedule then (choice C)?
You're right that group 2 had a fixed ratio reinforcement, but I think C is wrong because there is no group that has a variable ratio reinforcement schedule. Since the study doesn't mention or give data for this type of group, then we can't make any assumptions in the answer.