the graph also doesn't have enough information to conclude that the use of Bt corn will decrease environmental impact
The questions says to use the information in the graph
and your knowledge from the course to choose a valid statement.
The course does specify that we need to know a bit about the use of genetically modified crops are used in agriculture which one could intepreate as involving knowledge that GMOs can reduce pesticide/herbicide usage which has a positive impact on the environment. While I can't find this specific bit of content being assesed previously, It does seem to fit within the study design.
As such, using the graph to confirm that Bt crops reduce herbicide use, we can conclude that negative environmental impacts will be reduced due to a reduced use of herbicides/pesticides
On the otherhand, the study design does not make any reference to Bt crops specifically, so there would be no expectation to know how much they specifically cost compared to regular costs, thus you cannot conclude that Bt crops cost less to produce than regular crops.