I doubt you have to know the reasons for it in terms of its structure, graphene is a material with a lot of potential uses and has been getting a lot of attention recently. You just need to know that they are some of the properties of graphene.
Since you circled diamond and graphite i assume they were the two answers you were deciding between.
Diamond isn't light, flexible or a conductor so we can eliminate this.
Graphene is just a sling layer of carbon atoms whereas graphite is pretty much layers upon layers of graphene stacked on top of each other, so it makes sense that this makes it less flexible and makes it heavier than graphene.
I can explain in further detail if you like
