The conclusion is that people live longer if they exercise regularly.
A – Wrong
In order for he conclusion to be drawn, it is not necessary to assume that elderly and middle aged people have exercised throughout their lives.
B – Wrong
The passage does not indicate that people walk vigorously. So it is not necessarily needed to assume this.
C – Wrong
The passage does not assume that being overweight reduces lifespan. The passage just indicates a positive correlation between exercising regularly and longer lifespan. It does not necessarily need to assume a positive correlation between being overweight and reduced lifespan.
D – Correct
The text assumes that exercising is the main factor which extends lifespan and does not consider other factors.
E – Wrong
Nothing is suggested about exercising delaying heart disease.