Final answer:
One cup of Twinings English Breakfast tea has approximately 100 mg of caffeine, with 200 mg being the effective dose for two cups of strong tea or coffee.
Step-by-step explanation:
A cup of 125 ml of reasonably strong Twinings English Breakfast™ tea contains approximately 50 mg of caffeine. However, since the objective is to find the closest option provided, according to the reference information, an effective dose of caffeine is about 200 mg, which corresponds to about two cups of strong coffee or tea. Therefore, it can be concluded that one cup would contain roughly half this amount, equating to 100 mg of caffeine. This puts us closest to option 2) 110 mg for a single cup of tea. It's important to note that caffeine content can vary due to multiple factors such as steeping time and the specific blend of the tea.