After observing thirty-year-old albatrosses, biologists determined tha

30 year old albatrosses were observed.
It was found that those albatrosses that hatched alone or whose fellow fledglings did not survive after their first molting were observed to follow shorter migration paths after 30 years

Conclusion: Migration distance in mature albatrosses is directly tied to the lifespan of other fledglings in their brood

So the albatrosses whose fellow fledglings survived went longer distances than albatrosses whose fellow fledglings did not survive (or there weren't any fellow fledglings to begin with)

It follows that:

A. a few albatrosses who molted before maturity are capable of migrating further after thirty years as a result

Albatrosses who melted before maturity are not discussed in the argument.

B. the migration distance of thirty-year-old albatrosses is connected to the migration distance of other fledglings

The migration distance of other fledglings is not discussed. Their survival is discussed.

C. it is the survival of other fledglings rather than just the birth of those fledglings that more directly affects the migration distance of albatrosses after thirty years

Correct. The argument links migration distance to 'survival of the other fledglings', not the birth of other fledglings. The albatrosses who had no fellow fledglings did not fly far. The ones who had fellow fledglings (so birth happened) but the fledglings did not survive also do not fly far.
The albatrosses whose fellow fledglings survived flew far.

D. albatrosses that migrated long distances after their first molting are more likely to migrate long distances after thirty years as well

Migration distance after molting is not discussed. Only migration distance 30 yrs later is discussed.

E. the lifespan of other fledglings is more important to albatrosses after their first molting than it is after thirty years

The argument does not tell us whether lifespan of other fledglings is important to albatrosses after the albatrosses' first molting. It could be more or less important at that time to them. What we do know is that lifespan of other fledglings is important to albatrosses after 30 yrs.

Answer (C)