C. It is possible that some individuals could be naturally resistant to developing both type 2 diabetes and heart disease regardless of diet.
Eating non genetically modified food is not the reason to stay healthy (some are naturally resistant) INTERESTING (but it says some individuals, some can be those who eat genetically modified food and may be those who doesn't eat GMF)
Doesn't weaken the conclusion "eating only non genetically modified foods leads to a lower risk of heart disease and type 2 diabetes" E. Individuals who do not eat genetically modified foods also tend to exercise and make other healthy lifestyle choices associated with a decreased risk of both diseases.
Eating non genetically modified food is not the only reason to stay healthy, doing exercise and other healthy lifestyle choices are also the reasons.
Hence it weakens the conclusion.
Plus last para states "concluded that the decreased risk of both diseases
must be as a result of the individuals’ dietary differences." Hence eating non GMF is not the only reason.
IMO E is the correct answer