Final answer:
The substance that fits the description of being a red, highly reactive liquid used as a disinfectant is bromine. It is known for its reddish-brown color, fuming nature, and offensive smell.
Step-by-step explanation:
The substance described in the question as a red, highly reactive, fuming liquid with a foul smell that is used as a disinfectant is bromine (B). Bromine is a deep reddish-brown liquid at room temperature, which is distinguished by its high vapor pressure and the visibility of its reddish vapor. Its properties can be observed in typical representations of the halogens where bromine's liquid and vapor states are shown in an orange hue while other halogens like chlorine and iodine are in gaseous and solid states, respectively. Bromine's use as a disinfectant is limited due to its reactivity and the hazards associated with handling it.