Skip would like his partner to stop leaving shaving cream and stubble in the sink after he shaves in the morning. at first, skip complains to his partner about the mess. then he changes tactics: he his partner on the occasional mornings when he rinses out the sink. skip has changed from using to using a. negative reinforcement; b. positive reinforcement negative reinforcement; c. negative punishment positive reinforcement; d. negative reinforcement

Skip has changed from using positive punishment to using positive reinforcement.

Explanation:

When Skip complains and yells to his partner about the mess in the sink, he is applying positive punishment, the word positive doesn't refer to a "good" punishment, but to adding a punishment that would decrease the unwanted behavior. Skip thought that by adding a positive punishment (complaining) his partner would stop leaving shaving cream in the sink.

When he saw that it wasn't working he began to ignore the days in which the partner would leave the sink dirty and focus on the days in which he cleaned it up, to try to reinforce that behavior. He began to use positive reinforcement, by adding a pleasent stimulus (saying thank you), which would make the behavior more likely to be repeated.