The correct answer is serotonin.
Obsessive-compulsive disorder refers to a mental ailment in which the individuals do certain routines frequently, feel the requirement to check the things repeatedly or possess some kind of thoughts repeatedly.
Serotonin refers to a monoamine neurotransmitter and is well known as a contributor to the feelings of happiness and well-being. It is mainly found in the enteric nervous system situated in the GI tract. However, it is also generated in the CNS mainly in the Raphe nuclei situated in the brainstem.