Which mineral would be found in greatest concentration in the plasma of the marathon runners? a. zinc b. phosphorus c. potassium d. iron e. sodium?

The right answer is E. Sodium  (with 140mmol/L)

Sodium is the most important element in the blood and extracellular liquids of the body.The needs of an adult subject are of the order of 1 to 2 g of sodium per day. The normal diet is much richer than our actual needs, about 4g, so it can largely compensate for the needs of an athlete or a runner.
Losses are majoritarly by feces and sweat. The kidney is able to reabsorb almost all the sodium that is filtered: urinary losses can be tiny.