The correct answer is - a. They began to demand wages for their labor.
After the plague ended, the countries affected by it were in a very bad position because they lost millions of people because of it. That led to a very big deficit of laborers. The laborers peasants and serfs that survived found themselves in a situation where there was work in abundance, so much so that they were not even able to work on all farmlands. Because of that, they started to demand higher wages for their labor, and since the large land owners were lacking laborers and started competing for the ones that remained, the peasants and serfs started to get much more money for their work.