The correct answer is: a. “I appreciated your help today.”
Indeed, every human being in a society needs to learn its codes in order to properly function: this is called social interaction. In order to enhance social interaction, individuals are expected to exhibit social behavior. Social behavior is the behavior of an individual with regards to another and its effects will determine if the interaction is conflictive or amicable.
In the case of this sentence, it is meant to use verbal communication to, express the fact that the person who has received such help is positively impressed and that he intends to reciprocate the other party when the occasion arises. The other party, who has helped the person, expects such words, which provide verbal confirmation that his expectation of future reciprocity will be satisfied. Such behaviors enhances the chances of survival for the group.