Fish are given to dolphins several times a day; many dolphins get as much as half of their food each day there.
Dolphins that first benefit from the program as adults are healthy and long-lived, but their offspring have a lower life expectancy than offspring of dolphins that feed exclusively in the wild.

This is a kind of 'explain the paradox' question. You expect the offspring of all dophins to have similar life expectancy. You might even think that dolphins who feed at the beach and their offspring might, in fact, have better life expectancy than others since half of their daily need is supplied by the beach. But actually offspring of dolphins that feed at the beach have lower life expectancy!

What will explain this?

A. Sharks that prey on dolphins are less common in the open seas off the beach than in many other areas of the open seas where dolphins congregate.

If the number of predators is lower near the beach, it should increase the life expectancy of the dolphin offsprings, not decrease it. Not correct.

B. Many of the adult dolphins that feed at the beach are females that nurse their offspring there.

This should increase the life expectancy of the dolphin offsprings since they get half their food from the beach program. Not correct.

C. The fish given to the dolphins at the beach are the same types of fish that dolphins typically catch in the wild.

This also implies that the life expectancy of the dolphin offsprings should not be any lower. Not correct.

D. Many dolphins that feed at the beach with their offspring come to the beach only a few times a month.

This doesn't explain why the life expectancy of the offspring of these dolphins should be lower. Perhaps, if they come to the beach only a few times a month, the beach has no importance for them. But then, why should their offspring have lower life expectancy? The others make do with the fish from the wild - they can do so too.

E. Adult dolphins that feed at the beach spend much less time teaching their offspring how to catch fish in the wild than do other adult dolphins.

This explains the paradox perfectly. Adult dolphins that are busy at the beach don't teach fishing to their offspring. This means that the offspring don't learn how to make up the other half of their daily diet. The fish at the beach provide only half of their diet. They don't learn how to catch fish in the wild and make up the day's quota. On the other hand, the dolphins sticking to the wild spend time teaching fishing to their offspring and hence the life expectancy of their offspring is higher.

Answer (E)