It illustrates the popularity of tea in Britain during the 1800s.
In MacGregor's writing, he lists different facts to show teas popularity. He talks about it being the "new national drink" and "consumption...was a staggering 6 lbs." Geijer's comments show how many people were drinking tea as well. He shows how tea was popular among all social classes. He is supporting the description of tea's popularity. This eliminates options B and D immediately as they do not speak of the drink's popularity. The reason the answer is not A is because these details only show that tea is popular it does not show the way in which it became popular.