The following is a brainstorming technique she can use to narrow down the topic:
B.make a list of ideas related to genetically modified foods.
Explanation:
Brainstorming is supposed to be quick, direct, straight to the point. It aims at gathering as many words or ideas related to a specific topic in little time. Therefore, making a list of ideas related to genetically modified foods is the way to go if Lena is willing to use a brainstorming technique in order to narrow down the topic.