I also think that the answer is option B

What is POE?

(A) A substance from the licorice plant, 50 times sweeter than sucrose, was recently discovered
>>>>> 50 times sweeter... is incorrectly modifying plant
(B) A substance, which was recently discovered, from the licorice plant, 50 times sweeter than sucrose,
>>>>> 50 times sweeter ..is correctly modifying A substance

(C) A substance from the licorice plant, which was recently discovered to be 50 times sweeter than sucrose,
>>>>>>>> referring that plant was recently discovered
(D) A substance from the licorice plant, 50 times sweeter than sucrose , which was recently discovered,
>>>>>>>> referring sucrose was recently discovered
(E) A recently discovered substance, 50 times sweeter than sucrose from the licorice plant,
>>>>>> Referring sucrose from the ..plant

B..

POE = Process of elimination .... this is the process in which one searches for the issues, identification of which will lead to elimination of options. This process brings down the number of choices you are left to work with finally, elevating chances of educated guess which is the last option if one is left with more than one choice.