I was stuck between B and D and eventually chose B.

Here is my anaylysis.

The average life expectancy for the United States population as a whole is 73.9 years, but
children born in Hawaii will live an average of 77 years, and those born in Louisiana, 71.7
years. If a newlywed couple from Louisiana were to begin their family in Hawaii, therefore, their
children would be expected to live longer than would be the case if the family remained in
Louisiana.

US ALE = 73.9 yrs

H ALE (77 yrs) > L ALE (71.7 yrs)

If Lousinians start a family in Hawaii their children are expected to live longer in Hawaii than would have if they were born in Lousiana.

Answer paraprasing:

There is something apart from the biological factors that favour extra age of Hawaiins.

Which of the following statements, if true, would most significantly strengthen the conclusion
drawn in the passage?

A. As population density increases in Hawaii, life expectancy figures for that state are likely
to be revised downward. Weakens
B. Environmental factors tending to favor longevity are abundant in Hawaii and less
numerous in Louisiana.
Bingo.. this goes with what I have assumed to be my answer.
C. Twenty-five percent of all Louisianans who move to Hawaii live longer than 77 years.
Though relative but loosely conclusive.
D. Over the last decade, average life expectancy has risen at a higher rate for Louisianans
than for Hawaiians.
Out of scope, since the ALE increased during last decade and we have figures which include this rise.
E. Studies show that the average life expectancy for Hawaiians who move permanently to
Louisiana is roughly equal to that of Hawaiians who remain in Hawaii.
Weakens