Acommon and legal substance used by many athletes as an ergogenic aid and shown in some studies to increase exercise endurance is/are

There are a few possible answers for this.
Caffeine is used to increase exercise endurance by the athlete and it is legal. Caffeine could be found in tea or coffee. It has a mild diuretic effect that gives concern of the risk of dehydration.
Creatine phosphate also used to increase creatine in the muscle. Creatine is the first energy source that used by the muscle. It is easily depleted so it mainly used to increase endurance in high-intensity exercise. 

On edgenuity it is d: watermelons 

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