A. The ease of learning a second language depends almost exclusively on environmental factors. (As per the passage, The ease of learning a new language depends on the conditioning of child at an early age) Incorrect
B. A student whose parents only read one book a week to her only has a 25% chance of mastering a foreign language. (Their retention rate falls, but that doesn't mean they can't master foreign language) Incorrect.
C. Students whose parents enter them in early education and who read to them frequently are more likely to have extra income and more free time. (Income and Free time is not the concern here) Incorrect.
D. Proficient speakers of a second language are likely to have begun learning it before the age of 6. (Passage does talk about the advantage of learning before the age of 6) Keep this
E. Students who begin studying a language later in life would have had an easier time learning some aspects of that language if they had begun studying it as a young child. (Yes, it will be easier for people to comprehend, at least a part of a foreign language if they had learned it at an early age) Keep this.
Between D and E.
D states all proficient speakers must have started learning a new language before turning 6. But it's nowhere stated in the passage, that all proficient language speakers must learn it at young age. Yes, it helps, but mastering language is optional.
The answer is E as it states learning at an early age helps kids to learn more of a foreign language.
Posted from my mobile device