Alcohol

Reaction time is the time taken to give response to a stimulus. Alcohol increases reaction time i.e. the body takes more time to respond to a stimulus. It affects the ability of body to react in response to a stimulus. Alcohol affects brain due to which the region of brain associated with processing of nerve signals is affected. Signals travel in brain from one neuron to other neuron. Alcohol increases the time in which a signal crosses the gap between two neurons i.e. synapse. Hence, the ability of body to interpret, perceive and responding to stimulus is greatly affected.

So, the correct answer is 'Increases reaction time'.